/* ========================================
   Physics Formulas Quiz — Complete Stylesheet
   Modern, Quizlet-inspired design
   ======================================== */

/* ===== 0. CSS Variables & Reset ===== */
:root {
    --bg: #f5f7fa;
    --bg-subtle: #edf0f5;
    --card: #ffffff;
    --card-hover: #f0f4f8;
    --accent: #1e3a8a;
    --accent-light: #2d5bbf;
    --accent-dark: #152c6b;
    --accent-glow: rgba(30, 58, 138, 0.22);
    --gradient-accent: linear-gradient(135deg, #1e3a8a, #2d5bbf);
    --gradient-accent-h: linear-gradient(135deg, #152c6b, #1e3a8a);
    --text: #1a1a2e;
    --text-muted: #6b7280;
    --text-light: #9ca3af;
    --border: #dde1e8;
    --border-light: #eef0f4;
    --success: #22c55e;
    --success-dark: #16a34a;
    --error: #ef4444;
    --error-dark: #dc2626;
    --warning: #f59e0b;
    --warning-dark: #d97706;
    --radius: 20px;
    --radius-sm: 12px;
    --radius-xs: 8px;
    --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.04), 0 1px 2px rgba(0, 0, 0, 0.06);
    --shadow: 0 4px 16px rgba(0, 0, 0, 0.06), 0 1px 4px rgba(0, 0, 0, 0.04);
    --shadow-lg: 0 10px 40px rgba(0, 0, 0, 0.08), 0 2px 8px rgba(0, 0, 0, 0.04);
    --shadow-xl: 0 20px 60px rgba(0, 0, 0, 0.1), 0 4px 16px rgba(0, 0, 0, 0.06);
    --font: 'Segoe UI', system-ui, -apple-system, BlinkMacSystemFont, sans-serif;
    --transition: 0.25s cubic-bezier(0.4, 0, 0.2, 1);
    --transition-bounce: 0.4s cubic-bezier(0.34, 1.56, 0.64, 1);
}

*, *::before, *::after {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
    overflow-x: hidden;
}

body {
    font-family: var(--font);
    background: var(--bg);
    color: var(--text);
    font-size: 16px;
    line-height: 1.6;
    min-height: 100vh;
    padding-top: 72px;
    overflow-x: hidden;
    -webkit-text-size-adjust: 100%;
}

/* Prevent double-tap zoom on interactive elements */
button, a, input, select, label, .match-card, .choice-btn, .mode-btn {
    touch-action: manipulation;
}

/* ===== 1. Top Navigation Bar ===== */
.top-nav {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    height: 68px;
    background: rgba(255, 255, 255, 0.82);
    backdrop-filter: blur(16px) saturate(180%);
    -webkit-backdrop-filter: blur(16px) saturate(180%);
    border-bottom: 1px solid rgba(226, 229, 236, 0.6);
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0 28px;
    z-index: 1000;
    box-shadow: 0 1px 8px rgba(0, 0, 0, 0.04), 0 0 1px rgba(0, 0, 0, 0.06);
}

.nav-brand {
    display: flex;
    align-items: center;
    gap: 10px;
    text-decoration: none;
    flex-shrink: 0;
}

.nav-logo {
    font-size: 1.6rem;
    font-weight: 800;
    background: var(--gradient-accent);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    line-height: 1;
}

.nav-title {
    font-size: 1.05rem;
    font-weight: 700;
    color: var(--text);
    white-space: nowrap;
    letter-spacing: -0.01em;
}

.nav-stats {
    display: flex;
    align-items: center;
    gap: 14px;
    flex-shrink: 0;
}

.studied-count {
    font-size: 0.85rem;
    color: var(--text-muted);
    font-weight: 500;
    white-space: nowrap;
}

.studied-count strong {
    color: var(--text);
    font-weight: 700;
}

.mini-progress {
    width: 110px;
    height: 6px;
    background: var(--border);
    border-radius: 3px;
    overflow: hidden;
    position: relative;
}

.mini-progress-fill {
    height: 100%;
    background: var(--gradient-accent);
    border-radius: 3px;
    transition: width 0.5s cubic-bezier(0.4, 0, 0.2, 1);
    position: relative;
}

.mini-progress-fill::after {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: linear-gradient(90deg, transparent, rgba(255,255,255,0.3), transparent);
    animation: gradientShift 2s ease-in-out infinite;
}

.nav-right {
    display: flex;
    align-items: center;
    gap: 12px;
    flex-shrink: 0;
}

.streak {
    display: inline-flex;
    align-items: center;
    gap: 4px;
    padding: 5px 14px;
    font-weight: 700;
    font-size: 0.9rem;
    color: #fff;
    background: linear-gradient(135deg, #f59e0b, #ef4444);
    border-radius: 20px;
    white-space: nowrap;
    box-shadow: 0 2px 8px rgba(245, 158, 11, 0.3);
    transition: transform var(--transition), box-shadow var(--transition);
}

.streak::before {
    content: '\1F525';
    font-size: 1rem;
}

.streak:hover {
    transform: scale(1.05);
    box-shadow: 0 4px 14px rgba(245, 158, 11, 0.4);
}

.streak.active {
    animation: streakFlame 1.8s ease-in-out infinite;
}

.nav-btn {
    width: 44px;
    height: 44px;
    display: flex;
    align-items: center;
    justify-content: center;
    background: transparent;
    border: 1px solid transparent;
    border-radius: 50%;
    font-size: 1.3rem;
    color: var(--text-muted);
    cursor: pointer;
    transition: all var(--transition);
}

.nav-btn:hover {
    background: var(--card-hover);
    border-color: var(--border);
    color: var(--text);
    transform: rotate(30deg);
}

.nav-btn:active {
    transform: rotate(30deg) scale(0.92);
}

/* ===== 2. Main Content Area ===== */
.main-content {
    max-width: 920px;
    margin: 0 auto;
    padding: 28px 24px 72px;
}

/* ===== 3. Filter Row ===== */
.filter-row {
    display: flex;
    align-items: center;
    gap: 16px;
    margin-bottom: 24px;
    flex-wrap: wrap;
}

.section-filter {
    flex: 1;
    min-width: 200px;
    max-width: 360px;
    padding: 11px 40px 11px 16px;
    font-size: 0.92rem;
    font-family: var(--font);
    font-weight: 500;
    color: var(--text);
    background: var(--card);
    border: 1.5px solid var(--border);
    border-radius: var(--radius-sm);
    outline: none;
    cursor: pointer;
    transition: all var(--transition);
    appearance: none;
    -webkit-appearance: none;
    background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='12' height='7'%3E%3Cpath d='M1 1l5 5 5-5' stroke='%236b7280' fill='none' stroke-width='1.5' stroke-linecap='round'/%3E%3C/svg%3E");
    background-repeat: no-repeat;
    background-position: right 14px center;
    box-shadow: var(--shadow-sm);
}

.section-filter:focus {
    border-color: var(--accent);
    box-shadow: 0 0 0 3px var(--accent-glow), var(--shadow-sm);
}

.section-filter:hover {
    border-color: var(--accent-light);
}

.starred-filter {
    display: flex;
    align-items: center;
    gap: 10px;
    font-size: 0.9rem;
    font-weight: 500;
    color: var(--text-muted);
    cursor: pointer;
    user-select: none;
    white-space: nowrap;
}

/* Toggle switch for checkbox */
.starred-filter input[type="checkbox"] {
    position: relative;
    width: 44px;
    height: 24px;
    appearance: none;
    -webkit-appearance: none;
    background: var(--border);
    border-radius: 12px;
    cursor: pointer;
    transition: background var(--transition);
    flex-shrink: 0;
}

.starred-filter input[type="checkbox"]::after {
    content: '';
    position: absolute;
    top: 3px;
    left: 3px;
    width: 18px;
    height: 18px;
    background: #fff;
    border-radius: 50%;
    box-shadow: 0 1px 3px rgba(0,0,0,0.15);
    transition: transform var(--transition-bounce);
}

.starred-filter input[type="checkbox"]:checked {
    background: var(--gradient-accent);
}

.starred-filter input[type="checkbox"]:checked::after {
    transform: translateX(20px);
}

/* ===== 4. Mode Selector Buttons ===== */
.mode-selector {
    display: flex;
    gap: 10px;
    margin-bottom: 28px;
    padding: 4px;
    overflow-x: auto;
    scrollbar-width: none;
}

.mode-selector::-webkit-scrollbar {
    display: none;
}

.mode-btn {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 11px 20px;
    font-size: 0.92rem;
    font-weight: 600;
    font-family: var(--font);
    color: var(--text-muted);
    background: var(--card);
    border: 1.5px solid var(--border);
    border-radius: 50px;
    cursor: pointer;
    white-space: nowrap;
    transition: all var(--transition);
    box-shadow: var(--shadow-sm);
    position: relative;
    overflow: hidden;
}

.mode-btn::before {
    content: '';
    position: absolute;
    inset: 0;
    background: var(--gradient-accent);
    opacity: 0;
    transition: opacity var(--transition);
}

.mode-btn:hover {
    color: var(--accent);
    border-color: var(--accent-light);
    transform: translateY(-2px);
    box-shadow: var(--shadow);
}

.mode-btn.active {
    color: #fff;
    background: var(--gradient-accent);
    border-color: transparent;
    box-shadow: 0 4px 16px var(--accent-glow);
    transform: translateY(-1px);
}

.mode-btn.active:hover {
    box-shadow: 0 6px 24px var(--accent-glow);
    transform: translateY(-3px);
}

.mode-btn .mode-icon {
    font-size: 1.3rem;
    line-height: 1;
    position: relative;
    z-index: 1;
}

.mode-btn .mode-label {
    position: relative;
    z-index: 1;
}

/* ===== 5. Flashcard Mode ===== */
.flashcard-progress {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 12px;
    margin-bottom: 20px;
    font-size: 0.95rem;
    font-weight: 600;
    color: var(--text-muted);
}

.flashcard-container {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 16px;
    perspective: 1200px;
    margin: 0 auto 24px;
    position: relative;
}

.flashcard {
    width: 100%;
    max-width: 580px;
    min-height: 420px;
    cursor: pointer;
    position: relative;
}

.flashcard-inner {
    width: 100%;
    min-height: 420px;
    position: relative;
    transform-style: preserve-3d;
    transition: transform 0.6s cubic-bezier(0.4, 0, 0.2, 1);
}

.flashcard.flipped .flashcard-inner {
    transform: rotateY(180deg);
    animation: flipScale 0.6s cubic-bezier(0.4, 0, 0.2, 1);
}

.flashcard-front,
.flashcard-back {
    position: absolute;
    inset: 0;
    backface-visibility: hidden;
    -webkit-backface-visibility: hidden;
    border-radius: var(--radius);
    background: var(--card);
    border: 1px solid var(--border-light);
    box-shadow: var(--shadow-lg);
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 40px 32px;
    text-align: center;
    overflow-y: auto;
    overflow-x: hidden;
}

.flashcard-front {
    border-top: 4px solid transparent;
    border-image: var(--gradient-accent) 1;
    border-image-slice: 1;
    border-top-left-radius: var(--radius);
    border-top-right-radius: var(--radius);
    /* Fix for border-image + border-radius conflict */
    position: absolute;
    inset: 0;
}

/* Gradient top border via pseudo-element (since border-image breaks border-radius) */
.flashcard-front::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    height: 4px;
    background: var(--gradient-accent);
    border-radius: var(--radius) var(--radius) 0 0;
}

.flashcard-front {
    border-top: none;
    border-image: none;
}

.flashcard-back {
    transform: rotateY(180deg);
    justify-content: flex-start;
    padding-top: 36px;
}

.card-badge {
    display: inline-block;
    padding: 4px 14px;
    font-size: 0.72rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    border-radius: 20px;
    background: rgba(79, 110, 247, 0.08);
    color: var(--accent);
    border: 1px solid rgba(79, 110, 247, 0.15);
    margin-bottom: 16px;
}

.card-term {
    font-size: 1.45rem;
    font-weight: 700;
    color: var(--text);
    line-height: 1.4;
    max-width: 440px;
    margin-bottom: 8px;
}

.card-code {
    font-size: 0.82rem;
    font-weight: 500;
    color: var(--text-light);
    margin-top: 4px;
}

.flip-hint {
    position: absolute;
    bottom: 20px;
    font-size: 0.72rem;
    color: var(--text-light);
    font-weight: 400;
    opacity: 0.55;
    letter-spacing: 0.3px;
    animation: fadeInOutHint 3s ease-in-out infinite;
}

.card-formula {
    margin-bottom: 18px;
    width: 100%;
}

.card-notes {
    font-size: 0.88rem;
    color: var(--text-muted);
    max-width: 440px;
    line-height: 1.6;
    margin-bottom: 10px;
}

.card-units {
    font-size: 0.85rem;
    color: var(--text-muted);
    margin-top: 4px;
}

.card-units strong {
    color: var(--text);
}

.star-btn {
    position: absolute;
    top: 16px;
    right: 16px;
    background: none;
    border: none;
    font-size: 1.5rem;
    color: var(--border);
    cursor: pointer;
    z-index: 5;
    transition: color var(--transition), transform var(--transition-bounce);
    line-height: 1;
}

.star-btn:hover {
    transform: scale(1.25);
    color: var(--warning);
}

.star-btn.starred {
    color: var(--warning);
    filter: drop-shadow(0 0 8px rgba(245, 158, 11, 0.5));
    animation: starFillGold 0.5s cubic-bezier(0.34, 1.56, 0.64, 1);
}

.nav-arrow {
    width: 50px;
    height: 50px;
    border-radius: 50%;
    background: var(--card);
    border: 1.5px solid var(--border);
    font-size: 1.5rem;
    font-weight: 300;
    color: var(--text);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: all var(--transition);
    box-shadow: var(--shadow-sm);
    flex-shrink: 0;
}

.nav-arrow:hover {
    background: var(--accent);
    color: #fff;
    border-color: var(--accent);
    transform: scale(1.1);
    box-shadow: 0 4px 14px var(--accent-glow);
}

.nav-arrow:active {
    transform: scale(0.95);
}

.flashcard-actions {
    display: flex;
    justify-content: center;
    gap: 16px;
    margin-top: 4px;
}

.flashcard-actions .btn {
    min-width: 140px;
    padding: 14px 28px;
    font-size: 1rem;
}

/* ===== Card Example (on flashcard back) ===== */
.card-example {
    width: 100%;
    max-width: 460px;
    margin-top: 24px;
    padding: 20px 22px;
    background: rgba(79, 110, 247, 0.04);
    border: 1px solid rgba(79, 110, 247, 0.12);
    border-left: 3px solid var(--accent);
    border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
    position: relative;
}

.example-header {
    font-size: 0.82rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    color: var(--accent);
    margin-bottom: 10px;
}

.example-question {
    font-size: 0.92rem;
    color: var(--text);
    line-height: 1.5;
    margin-bottom: 12px;
}

.example-answer-row {
    display: flex;
    gap: 8px;
    margin-bottom: 10px;
}

.example-input {
    flex: 1;
    padding: 9px 14px;
    font-size: 0.9rem;
    font-family: var(--font);
    border: 1.5px solid var(--border);
    border-radius: var(--radius-xs);
    outline: none;
    transition: all var(--transition);
    background: var(--card);
}

.example-input:focus {
    border-color: var(--accent);
    box-shadow: 0 0 0 3px var(--accent-glow);
}

.example-feedback {
    font-size: 0.88rem;
    font-weight: 600;
    padding: 8px 12px;
    border-radius: var(--radius-xs);
    animation: fadeIn 0.3s ease;
}

.example-feedback:empty {
    display: none;
}

.example-feedback.correct {
    color: var(--success-dark);
    background: rgba(34, 197, 94, 0.08);
}

.example-feedback.correct::before {
    content: '\2713  ';
}

.example-feedback.wrong {
    color: var(--error-dark);
    background: rgba(239, 68, 68, 0.08);
}

.example-feedback.wrong::before {
    content: '\2717  ';
}

.example-solution {
    margin-top: 10px;
    padding: 10px 14px;
    background: rgba(79, 110, 247, 0.06);
    border-radius: var(--radius-xs);
    font-size: 0.88rem;
    color: var(--text);
    line-height: 1.6;
}

/* ===== 6. Learn Mode (Leitner) ===== */
.leitner-boxes {
    display: flex;
    gap: 8px;
    margin-bottom: 24px;
    padding: 16px 18px;
    background: var(--card);
    border-radius: var(--radius-sm);
    border: 1px solid var(--border-light);
    box-shadow: var(--shadow-sm);
}

.leitner-box {
    flex: 1;
    text-align: center;
    padding: 10px 6px;
    border-radius: var(--radius-xs);
    font-weight: 700;
    color: #fff;
    cursor: default;
    transition: transform var(--transition-bounce), box-shadow var(--transition);
    position: relative;
}

.leitner-box:hover {
    transform: translateY(-3px);
    box-shadow: 0 4px 12px rgba(0,0,0,0.12);
}

.leitner-box.active {
    outline: 3px solid rgba(255, 255, 255, 0.7);
    outline-offset: 2px;
    box-shadow: 0 0 0 6px rgba(79, 110, 247, 0.3), 0 4px 12px rgba(0,0,0,0.12);
    animation: leitnerPulse 1.5s ease-in-out infinite;
    transform: translateY(-3px);
}

/* 5 colored boxes: red -> orange -> yellow -> light-green -> green */
.leitner-box[data-box="1"] {
    background: linear-gradient(135deg, #ef4444, #f87171);
}
.leitner-box[data-box="2"] {
    background: linear-gradient(135deg, #f97316, #fb923c);
}
.leitner-box[data-box="3"] {
    background: linear-gradient(135deg, #f59e0b, #fbbf24);
}
.leitner-box[data-box="4"] {
    background: linear-gradient(135deg, #84cc16, #a3e635);
    color: var(--text);
}
.leitner-box[data-box="5"] {
    background: linear-gradient(135deg, #22c55e, #4ade80);
}

.box-label {
    display: block;
    font-size: 0.7rem;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    opacity: 0.85;
    margin-bottom: 2px;
}

.box-count {
    display: block;
    font-size: 1.3rem;
    font-weight: 800;
}

.learn-card {
    max-width: 580px;
    margin: 0 auto;
    background: var(--card);
    border: 1px solid var(--border-light);
    border-radius: var(--radius);
    box-shadow: var(--shadow-lg);
    padding: 44px 32px;
    text-align: center;
}

.learn-term {
    font-size: 1.4rem;
    font-weight: 700;
    color: var(--text);
    margin-bottom: 8px;
    line-height: 1.4;
}

.learn-code {
    font-size: 0.82rem;
    color: var(--text-light);
    margin-bottom: 28px;
}

.learn-answer {
    margin-top: 20px;
    padding: 20px;
    background: var(--card-hover);
    border-radius: var(--radius-sm);
    animation: fadeIn 0.4s ease;
}

.learn-example {
    margin-top: 12px;
    animation: fadeIn 0.4s ease;
}

.learn-actions {
    display: flex;
    justify-content: center;
    gap: 16px;
    margin-top: 28px;
    animation: fadeIn 0.3s ease;
}

.learn-actions .btn {
    min-width: 160px;
    padding: 16px 32px;
    font-size: 1.05rem;
}

.learn-stats {
    display: flex;
    justify-content: center;
    gap: 28px;
    margin-top: 20px;
    font-size: 0.88rem;
    color: var(--text-muted);
}

.learn-stats span strong {
    color: var(--text);
    font-weight: 700;
}

.learn-empty {
    text-align: center;
    padding: 48px 24px;
    animation: fadeIn 0.5s ease;
}

.learn-empty h3 {
    font-size: 1.3rem;
    color: var(--text);
    margin-bottom: 10px;
}

.learn-empty p {
    color: var(--text-muted);
    margin-bottom: 24px;
}

/* ===== 7. Write Mode ===== */
.write-progress {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 12px;
    margin-bottom: 20px;
    font-size: 0.95rem;
    font-weight: 600;
    color: var(--text-muted);
}

.write-score {
    margin-left: 16px;
    color: var(--success-dark);
}

.write-card {
    max-width: 580px;
    margin: 0 auto;
    background: var(--card);
    border: 1px solid var(--border-light);
    border-radius: var(--radius);
    box-shadow: var(--shadow-lg);
    padding: 44px 32px;
    text-align: center;
}

.write-term {
    font-size: 1.35rem;
    font-weight: 700;
    color: var(--text);
    margin-bottom: 8px;
}

.write-code {
    font-size: 0.82rem;
    color: var(--text-light);
    margin-bottom: 28px;
}

.write-input-row {
    display: flex;
    gap: 10px;
    margin-bottom: 8px;
}

.write-input {
    flex: 1;
    padding: 14px 18px;
    font-size: 1.1rem;
    font-family: var(--font);
    border: 2px solid var(--border);
    border-radius: var(--radius-sm);
    outline: none;
    transition: all var(--transition);
    text-align: center;
    background: var(--card);
}

.write-input::placeholder {
    color: var(--text-light);
    transition: opacity var(--transition);
}

.write-input:focus::placeholder {
    opacity: 0.4;
}

.write-input:focus {
    border-color: var(--accent);
    box-shadow: 0 0 0 4px var(--accent-glow);
}

.write-input.correct {
    border-color: var(--success);
    background: rgba(34, 197, 94, 0.04);
    box-shadow: 0 0 0 4px rgba(34, 197, 94, 0.15);
    animation: correctFlash 0.6s ease;
}

.write-input.wrong {
    border-color: var(--error);
    background: rgba(239, 68, 68, 0.04);
    box-shadow: 0 0 0 4px rgba(239, 68, 68, 0.15);
    animation: shake 0.5s ease;
}

.write-feedback {
    margin-top: 16px;
    text-align: center;
    font-size: 0.95rem;
    font-weight: 600;
    padding: 12px 16px;
    border-radius: var(--radius-xs);
    animation: fadeIn 0.3s ease;
}

.write-feedback:empty {
    display: none;
}

.write-feedback.correct {
    color: var(--success-dark);
    background: rgba(34, 197, 94, 0.08);
}

.write-feedback.correct::before {
    content: '\2713  ';
}

.write-feedback.wrong {
    color: var(--error-dark);
    background: rgba(239, 68, 68, 0.08);
}

.write-feedback.wrong::before {
    content: '\2717  ';
}

.write-correct-answer {
    margin-top: 14px;
    padding: 14px 18px;
    background: var(--card-hover);
    border-radius: var(--radius-xs);
    font-size: 0.95rem;
    animation: slideUp 0.4s ease;
    border-left: 3px solid var(--accent);
}

/* ===== 8. Match Mode ===== */
.match-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 20px;
    padding: 14px 20px;
    background: var(--card);
    border-radius: var(--radius-sm);
    border: 1px solid var(--border-light);
    box-shadow: var(--shadow-sm);
}

.match-timer {
    display: flex;
    align-items: center;
    gap: 6px;
}

.timer-label {
    font-size: 0.85rem;
    color: var(--text-muted);
    font-weight: 500;
}

.timer-value {
    font-size: 2rem;
    font-weight: 800;
    font-family: 'Courier New', 'Consolas', monospace;
    color: var(--accent);
    min-width: 70px;
    background: rgba(79, 110, 247, 0.08);
    padding: 4px 12px;
    border-radius: var(--radius-xs);
    letter-spacing: 1px;
    text-align: center;
}

.match-best {
    font-size: 0.88rem;
    color: var(--text-muted);
}

.match-best strong {
    color: var(--accent);
    font-weight: 700;
}

.match-grid {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 12px;
}

.match-card {
    background: var(--card);
    border: 2px solid var(--border);
    border-radius: var(--radius-sm);
    padding: 18px 12px;
    text-align: center;
    font-size: 0.88rem;
    font-weight: 500;
    cursor: pointer;
    transition: all var(--transition);
    min-height: 80px;
    display: flex;
    align-items: center;
    justify-content: center;
    user-select: none;
    box-shadow: var(--shadow-sm);
}

.match-card:hover {
    border-color: var(--accent-light);
    background: var(--card-hover);
    transform: translateY(-2px);
    box-shadow: var(--shadow);
}

.match-card.selected {
    border-color: var(--accent);
    box-shadow: 0 0 0 3px var(--accent-glow), var(--shadow);
    background: rgba(79, 110, 247, 0.04);
    transform: translateY(-2px);
}

.match-card.matched {
    animation: correctFlash 0.4s ease, matchDisappear 0.5s 0.4s ease forwards;
    pointer-events: none;
    border-color: var(--success);
}

.match-card.wrong {
    animation: wrongFlash 0.5s ease, shake 0.5s ease;
    border-color: var(--error);
}

.match-complete {
    text-align: center;
    padding: 48px 24px;
    background: var(--card);
    border-radius: var(--radius);
    box-shadow: var(--shadow-lg);
    margin-top: 24px;
    animation: fadeIn 0.5s ease;
}

.match-complete h3 {
    font-size: 1.6rem;
    margin-bottom: 10px;
    background: var(--gradient-accent);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.match-complete p {
    color: var(--text-muted);
    font-size: 1.05rem;
    margin-bottom: 24px;
}

/* ===== 9. Test Mode ===== */
.test-setup {
    max-width: 480px;
    margin: 0 auto;
    background: var(--card);
    border-radius: var(--radius);
    box-shadow: var(--shadow-lg);
    padding: 40px 32px;
    text-align: center;
    animation: fadeIn 0.4s ease;
}

.test-setup h3 {
    font-size: 1.3rem;
    margin-bottom: 24px;
    color: var(--text);
}

.test-setting {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 12px;
    margin-bottom: 28px;
    font-size: 0.95rem;
    color: var(--text);
}

.test-setting label {
    font-weight: 500;
}

.test-setting select {
    padding: 8px 14px;
    font-size: 0.95rem;
    font-family: var(--font);
    border: 1.5px solid var(--border);
    border-radius: var(--radius-xs);
    background: var(--card);
    color: var(--text);
    cursor: pointer;
    outline: none;
    transition: border-color var(--transition);
}

.test-setting select:focus {
    border-color: var(--accent);
}

.test-questions {
    display: flex;
    flex-direction: column;
    gap: 20px;
}

.test-question {
    background: var(--card);
    border: 1px solid var(--border-light);
    border-radius: var(--radius);
    padding: 24px 28px;
    box-shadow: var(--shadow);
    animation: fadeIn 0.3s ease;
}

.test-question .question-number {
    font-size: 0.78rem;
    font-weight: 700;
    color: var(--accent);
    text-transform: uppercase;
    letter-spacing: 0.6px;
    margin-bottom: 10px;
}

.test-question .question-text {
    font-size: 1.08rem;
    font-weight: 600;
    margin-bottom: 18px;
    line-height: 1.5;
    color: var(--text);
}

.choices-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 10px;
}

/* The JS creates these as .choice-btn */
.choice-btn,
.test-choice {
    padding: 13px 18px;
    font-size: 0.95rem;
    font-family: var(--font);
    text-align: left;
    background: var(--card);
    border: 1.5px solid var(--border);
    border-radius: var(--radius-sm);
    cursor: pointer;
    transition: all var(--transition);
    color: var(--text);
}

.choice-btn:hover,
.test-choice:hover {
    background: var(--card-hover);
    border-color: var(--accent-light);
    transform: translateY(-1px);
}

.choice-btn.selected,
.test-choice.selected {
    border-color: var(--accent);
    background: rgba(79, 110, 247, 0.06);
    color: var(--accent-dark);
    font-weight: 600;
    box-shadow: 0 0 0 2px var(--accent-glow);
}

.choice-btn.correct-choice,
.test-choice.correct {
    border-color: var(--success);
    background: rgba(34, 197, 94, 0.08);
    color: var(--success-dark);
    font-weight: 600;
}

.choice-btn.wrong-choice,
.test-choice.wrong {
    border-color: var(--error);
    background: rgba(239, 68, 68, 0.08);
    color: var(--error-dark);
}

.tf-buttons {
    display: flex;
    gap: 12px;
}

.tf-buttons .choice-btn {
    flex: 1;
    text-align: center;
    font-weight: 600;
}

.test-input {
    width: 100%;
    padding: 12px 16px;
    font-size: 1rem;
    font-family: var(--font);
    border: 1.5px solid var(--border);
    border-radius: var(--radius-sm);
    outline: none;
    transition: all var(--transition);
    background: var(--card);
    color: var(--text);
}

.test-input:focus {
    border-color: var(--accent);
    box-shadow: 0 0 0 3px var(--accent-glow);
}

.matching-section {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 12px 24px;
}

.matching-section .match-item {
    padding: 10px 14px;
    background: var(--card-hover);
    border: 1.5px solid var(--border);
    border-radius: var(--radius-xs);
    font-size: 0.9rem;
    cursor: pointer;
    transition: all var(--transition);
}

.matching-section .match-item.selected {
    border-color: var(--accent);
    background: rgba(79, 110, 247, 0.06);
}

.matching-section .match-item.connected {
    border-color: var(--success);
    background: rgba(34, 197, 94, 0.06);
}

.test-submit-row {
    display: flex;
    justify-content: center;
    margin-top: 28px;
}

.test-results {
    max-width: 680px;
    margin: 0 auto;
    animation: fadeIn 0.5s ease;
}

.test-results h3 {
    text-align: center;
    font-size: 1.3rem;
    margin-bottom: 16px;
}

.test-score {
    text-align: center;
    margin-bottom: 28px;
    padding: 28px 24px;
    background: var(--card);
    border-radius: var(--radius);
    box-shadow: var(--shadow);
    position: relative;
    overflow: hidden;
}

.test-score-bar {
    position: absolute;
    bottom: 0;
    left: 0;
    height: 6px;
    border-radius: 0 3px 3px 0;
    background: var(--gradient-accent);
    transition: width 1s cubic-bezier(0.4, 0, 0.2, 1);
}

.test-score-circle {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 96px;
    height: 96px;
    border-radius: 50%;
    font-size: 1.8rem;
    font-weight: 800;
    margin-bottom: 12px;
    position: relative;
}

.test-score-circle.score-high {
    background: rgba(34, 197, 94, 0.1);
    color: var(--success-dark);
    box-shadow: 0 0 0 4px rgba(34, 197, 94, 0.2);
}

.test-score-circle.score-mid {
    background: rgba(245, 158, 11, 0.1);
    color: var(--warning-dark);
    box-shadow: 0 0 0 4px rgba(245, 158, 11, 0.2);
}

.test-score-circle.score-low {
    background: rgba(239, 68, 68, 0.1);
    color: var(--error-dark);
    box-shadow: 0 0 0 4px rgba(239, 68, 68, 0.2);
}

.test-review {
    display: flex;
    flex-direction: column;
    gap: 10px;
    margin-bottom: 24px;
}

/* ===== 10. Common Components ===== */

/* Buttons */
.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: 8px;
    padding: 12px 24px;
    font-size: 0.95rem;
    font-weight: 600;
    font-family: var(--font);
    border: none;
    border-radius: var(--radius-sm);
    cursor: pointer;
    transition: all var(--transition);
    position: relative;
    overflow: hidden;
    text-decoration: none;
    line-height: 1.4;
}

.btn:active {
    transform: scale(0.97);
}

.btn-primary {
    background: var(--gradient-accent);
    color: #fff;
    box-shadow: 0 2px 8px var(--accent-glow);
}

.btn-primary:hover {
    background: var(--gradient-accent-h);
    box-shadow: 0 4px 16px var(--accent-glow);
    transform: translateY(-1px);
}

.btn-primary:active {
    transform: translateY(0) scale(0.97);
}

.btn-success {
    background: linear-gradient(135deg, #22c55e, #16a34a);
    color: #fff;
    box-shadow: 0 2px 8px rgba(34, 197, 94, 0.25);
}

.btn-success:hover {
    background: linear-gradient(135deg, #16a34a, #15803d);
    box-shadow: 0 4px 16px rgba(34, 197, 94, 0.35);
    transform: translateY(-1px);
}

.btn-danger {
    background: linear-gradient(135deg, #ef4444, #dc2626);
    color: #fff;
    box-shadow: 0 2px 8px rgba(239, 68, 68, 0.25);
}

.btn-danger:hover {
    background: linear-gradient(135deg, #dc2626, #b91c1c);
    box-shadow: 0 4px 16px rgba(239, 68, 68, 0.35);
    transform: translateY(-1px);
}

.btn-outline {
    background: transparent;
    color: var(--accent);
    border: 2px solid var(--accent);
    padding: 10px 22px;
}

.btn-outline:hover {
    background: var(--accent);
    color: #fff;
    box-shadow: 0 4px 12px var(--accent-glow);
    transform: translateY(-1px);
}

.btn-sm {
    padding: 8px 16px;
    font-size: 0.84rem;
    border-radius: var(--radius-xs);
}

.btn-lg {
    padding: 16px 32px;
    font-size: 1.05rem;
    border-radius: var(--radius-sm);
}

/* Badge */
.badge {
    display: inline-block;
    padding: 3px 12px;
    font-size: 0.72rem;
    font-weight: 600;
    border-radius: 20px;
    background: var(--card-hover);
    color: var(--text-muted);
    border: 1px solid var(--border);
    letter-spacing: 0.3px;
}

.badge.badge-accent {
    background: rgba(79, 110, 247, 0.1);
    color: var(--accent);
    border-color: rgba(79, 110, 247, 0.2);
}

.badge.badge-success {
    background: rgba(34, 197, 94, 0.1);
    color: var(--success);
    border-color: rgba(34, 197, 94, 0.2);
}

/* Modal */
.modal {
    position: fixed;
    inset: 0;
    background: rgba(0, 0, 0, 0.4);
    backdrop-filter: blur(6px);
    -webkit-backdrop-filter: blur(6px);
    display: flex;
    align-items: center;
    justify-content: center;
    z-index: 2000;
    animation: fadeIn 0.25s ease;
    padding: 20px;
}

.modal-content {
    background: var(--card);
    border-radius: var(--radius);
    box-shadow: var(--shadow-xl);
    width: 100%;
    max-width: 480px;
    max-height: 85vh;
    overflow-y: auto;
    padding: 36px 32px;
    animation: slideUp 0.35s cubic-bezier(0.34, 1.56, 0.64, 1);
}

.modal-content h3 {
    font-size: 1.3rem;
    margin-bottom: 24px;
    color: var(--text);
}

.setting-label {
    display: flex;
    align-items: center;
    gap: 12px;
    font-size: 0.95rem;
    font-weight: 500;
    color: var(--text);
    margin-bottom: 16px;
    cursor: pointer;
}

.setting-label input[type="checkbox"] {
    width: 18px;
    height: 18px;
    accent-color: var(--accent);
    cursor: pointer;
}

.setting-row {
    margin-bottom: 20px;
    padding-top: 8px;
}

/* ===== 11. Animations ===== */
@keyframes shake {
    0%, 100% { transform: translateX(0); }
    15%      { transform: translateX(-8px); }
    30%      { transform: translateX(8px); }
    45%      { transform: translateX(-6px); }
    60%      { transform: translateX(6px); }
    75%      { transform: translateX(-3px); }
    90%      { transform: translateX(3px); }
}

@keyframes correctFlash {
    0%   { background-color: rgba(34, 197, 94, 0); }
    40%  { background-color: rgba(34, 197, 94, 0.15); }
    100% { background-color: rgba(34, 197, 94, 0); }
}

@keyframes wrongFlash {
    0%   { background-color: rgba(239, 68, 68, 0); }
    40%  { background-color: rgba(239, 68, 68, 0.15); }
    100% { background-color: rgba(239, 68, 68, 0); }
}

@keyframes matchDisappear {
    0%   { transform: scale(1); opacity: 1; }
    100% { transform: scale(0); opacity: 0; }
}

@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(12px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

@keyframes slideUp {
    from {
        opacity: 0;
        transform: translateY(24px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

@keyframes pulse {
    0%, 100% { transform: scale(1); }
    50%      { transform: scale(1.06); }
}

@keyframes flipScale {
    0%   { transform: rotateY(0deg) scale(1); }
    50%  { transform: rotateY(90deg) scale(0.95); }
    100% { transform: rotateY(180deg) scale(1); }
}

@keyframes gradientShift {
    0%   { transform: translateX(-100%); }
    100% { transform: translateX(200%); }
}

@keyframes streakFlame {
    0%, 100% {
        filter: drop-shadow(0 0 4px rgba(245, 158, 11, 0.3));
        transform: scale(1);
    }
    50% {
        filter: drop-shadow(0 0 12px rgba(245, 158, 11, 0.6));
        transform: scale(1.08);
    }
}


/* ===== 12. KaTeX Overrides ===== */
.katex {
    font-size: 1.2em;
    color: var(--text);
}

.flashcard-back .katex,
.card-formula .katex {
    font-size: 2.2em;
}

.match-card .katex {
    font-size: 1.1em;
}

.learn-card .katex,
.learn-answer .katex {
    font-size: 1.7em;
}

.write-correct-answer .katex {
    font-size: 1.3em;
}

/* ===== 13. Custom Scrollbar ===== */
::-webkit-scrollbar {
    width: 6px;
    height: 6px;
}

::-webkit-scrollbar-track {
    background: transparent;
}

::-webkit-scrollbar-thumb {
    background: var(--accent-light);
    border-radius: 3px;
}

::-webkit-scrollbar-thumb:hover {
    background: var(--accent);
}

/* Firefox */
* {
    scrollbar-width: thin;
    scrollbar-color: var(--accent-light) transparent;
}

/* ===== 14. Responsive - Tablet (768px) ===== */
@media (max-width: 768px) {
    body {
        padding-top: 64px;
    }

    .top-nav {
        height: 60px;
        padding: 0 16px;
    }

    .nav-title {
        font-size: 0.92rem;
    }

    .mini-progress {
        width: 80px;
    }

    .main-content {
        padding: 20px 14px 56px;
    }

    .mode-selector {
        gap: 6px;
        margin-bottom: 20px;
    }

    .mode-btn {
        padding: 10px 14px;
        font-size: 0.85rem;
        min-height: 44px;
        min-width: 44px;
        justify-content: center;
    }

    .mode-btn .mode-label {
        display: none;
    }

    .mode-btn .mode-icon {
        font-size: 1.4rem;
    }

    .flashcard {
        min-height: 340px;
    }

    .flashcard-inner {
        min-height: 340px;
    }

    .flashcard-front,
    .flashcard-back {
        padding: 28px 20px;
    }

    .card-term {
        font-size: 1.2rem;
    }

    .match-grid {
        grid-template-columns: repeat(3, 1fr);
    }

    .choices-grid {
        grid-template-columns: 1fr;
    }

    .matching-section {
        grid-template-columns: 1fr;
    }

    .modal-content {
        padding: 28px 22px;
    }

    .flashcard-container {
        gap: 10px;
    }

    .nav-arrow {
        width: 44px;
        height: 44px;
        font-size: 1.3rem;
    }

    .flashcard-actions .btn {
        min-width: 120px;
        padding: 12px 20px;
        font-size: 0.92rem;
    }
}

/* ===== 15. Responsive - Mobile (480px) ===== */
@media (max-width: 480px) {
    body {
        padding-top: 56px;
    }

    .top-nav {
        height: 56px;
        padding: 0 12px;
    }

    .nav-stats {
        display: none;
    }

    .nav-title {
        font-size: 0.85rem;
        max-width: 140px;
        overflow: hidden;
        text-overflow: ellipsis;
    }

    .nav-logo {
        font-size: 1.3rem;
    }

    .streak {
        padding: 6px 12px;
        font-size: 0.82rem;
        min-height: 44px;
        min-width: 44px;
        justify-content: center;
    }

    .nav-btn {
        width: 44px;
        height: 44px;
        font-size: 1.15rem;
    }

    .main-content {
        padding: 14px 10px 48px;
    }

    .filter-row {
        flex-direction: column;
        align-items: stretch;
        gap: 10px;
    }

    .section-filter {
        max-width: 100%;
    }

    .mode-selector {
        gap: 6px;
    }

    .mode-btn {
        padding: 10px 14px;
        font-size: 0.82rem;
        border-radius: 40px;
        min-height: 44px;
        min-width: 44px;
        justify-content: center;
    }

    .mode-btn .mode-icon {
        font-size: 1.3rem;
    }

    .flashcard {
        min-height: 280px;
    }

    .flashcard-inner {
        min-height: 280px;
    }

    .flashcard-front,
    .flashcard-back {
        padding: 24px 16px;
    }

    .card-term {
        font-size: 1.1rem;
    }

    .nav-arrow {
        width: 44px;
        height: 44px;
        font-size: 1.2rem;
    }

    .flashcard-actions {
        flex-direction: column;
        align-items: stretch;
        gap: 10px;
    }

    .flashcard-actions .btn {
        min-width: auto;
        min-height: 44px;
    }

    .match-grid {
        grid-template-columns: repeat(2, 1fr);
        gap: 8px;
    }

    .match-card {
        padding: 12px 8px;
        font-size: 0.82rem;
        min-height: 64px;
    }

    .timer-value {
        font-size: 1.4rem;
    }

    .match-header {
        padding: 10px 14px;
        flex-wrap: wrap;
        gap: 8px;
    }

    .learn-card,
    .write-card {
        padding: 28px 18px;
    }

    .learn-term,
    .write-term {
        font-size: 1.15rem;
    }

    .leitner-boxes {
        padding: 12px 10px;
        gap: 5px;
    }

    .leitner-box {
        padding: 8px 3px;
    }

    .box-label {
        font-size: 0.62rem;
    }

    .box-count {
        font-size: 1.05rem;
    }

    .test-question {
        padding: 18px 16px;
    }

    .test-setup {
        padding: 28px 20px;
    }

    .btn {
        padding: 10px 18px;
        font-size: 0.88rem;
        min-height: 44px;
    }

    .btn-lg {
        padding: 14px 24px;
        font-size: 0.95rem;
    }

    .learn-actions {
        flex-direction: column;
        align-items: stretch;
    }

    .learn-actions .btn {
        min-width: auto;
    }

    .write-input-row {
        flex-direction: column;
    }

    .write-input {
        font-size: 1rem;
        min-height: 44px;
    }

    .choice-btn,
    .test-choice {
        min-height: 44px;
        padding: 12px 14px;
        font-size: 0.88rem;
    }

    .test-input {
        min-height: 44px;
    }

    .section-filter {
        min-height: 44px;
    }

    .flashcard-back .katex,
    .card-formula .katex {
        font-size: 1.6em;
    }

    .learn-card .katex,
    .learn-answer .katex {
        font-size: 1.3em;
    }

    .example-answer-row {
        flex-direction: column;
    }

    .tf-buttons {
        flex-direction: column;
        gap: 8px;
    }
}

/* ===== 16. Mode content visibility helper ===== */
.mode-content {
    animation: fadeIn 0.35s ease;
}

/* ===== 17. Selection color ===== */
::selection {
    background: rgba(79, 110, 247, 0.2);
    color: var(--text);
}

/* ===== 18. Focus visible for accessibility ===== */
:focus-visible {
    outline: 2px solid var(--accent);
    outline-offset: 2px;
}

button:focus:not(:focus-visible),
input:focus:not(:focus-visible),
select:focus:not(:focus-visible) {
    outline: none;
}

/* ===== 19. Button Disabled States ===== */
.btn:disabled,
.btn[disabled],
.btn.disabled {
    opacity: 0.5;
    cursor: not-allowed;
    pointer-events: none;
    box-shadow: none;
    transform: none;
}

.mode-btn:disabled,
.mode-btn[disabled] {
    opacity: 0.4;
    cursor: not-allowed;
    pointer-events: none;
}

.nav-arrow:disabled,
.nav-arrow[disabled] {
    opacity: 0.35;
    cursor: not-allowed;
    pointer-events: none;
    box-shadow: none;
}

.match-card:disabled,
.match-card[disabled],
.match-card.disabled {
    opacity: 0.4;
    cursor: not-allowed;
    pointer-events: none;
}

.choice-btn:disabled,
.choice-btn[disabled],
.test-choice:disabled,
.test-choice[disabled] {
    opacity: 0.5;
    cursor: not-allowed;
    pointer-events: none;
}

/* ===== 20. Additional Interactive States ===== */

/* Active (pressed) states for elements missing them */
.mode-btn:active:not(.active) {
    transform: translateY(0) scale(0.96);
    box-shadow: var(--shadow-sm);
}

.match-card:active {
    transform: translateY(0) scale(0.97);
}

.choice-btn:active,
.test-choice:active {
    transform: translateY(0) scale(0.98);
    box-shadow: inset 0 1px 3px rgba(0, 0, 0, 0.1);
}

.star-btn:active {
    transform: scale(0.85);
}

.section-filter:active {
    transform: scale(0.99);
}

.starred-filter input[type="checkbox"]:active::after {
    width: 22px;
}

.btn-success:active {
    transform: translateY(0) scale(0.97);
}

.btn-danger:active {
    transform: translateY(0) scale(0.97);
}

.btn-outline:active {
    transform: translateY(0) scale(0.97);
}

/* Focus-visible enhancements for specific elements */
.mode-btn:focus-visible {
    outline: 2px solid var(--accent);
    outline-offset: 2px;
    box-shadow: 0 0 0 4px var(--accent-glow);
}

.match-card:focus-visible {
    outline: 2px solid var(--accent);
    outline-offset: 2px;
    box-shadow: 0 0 0 4px var(--accent-glow);
}

.star-btn:focus-visible {
    outline: 2px solid var(--accent);
    outline-offset: 2px;
}

.nav-arrow:focus-visible {
    outline: 2px solid var(--accent);
    outline-offset: 2px;
    box-shadow: 0 0 0 4px var(--accent-glow);
}

.choice-btn:focus-visible,
.test-choice:focus-visible {
    outline: 2px solid var(--accent);
    outline-offset: 2px;
    box-shadow: 0 0 0 3px var(--accent-glow);
}

.section-filter:focus-visible {
    border-color: var(--accent);
    box-shadow: 0 0 0 3px var(--accent-glow), var(--shadow-sm);
}

.starred-filter input[type="checkbox"]:focus-visible {
    box-shadow: 0 0 0 3px var(--accent-glow);
}

/* ===== 21. Visual Hierarchy Improvements ===== */

/* Stronger heading distinction in test results and learn empty */
.test-results h3,
.match-complete h3,
.learn-empty h3 {
    font-size: 1.4rem;
    font-weight: 800;
    letter-spacing: -0.02em;
    margin-bottom: 12px;
}

.test-setup h3,
.modal-content h3 {
    font-size: 1.3rem;
    font-weight: 700;
    letter-spacing: -0.01em;
}

/* Clearer section separators */
.filter-row {
    padding-bottom: 20px;
    border-bottom: 1px solid var(--border-light);
}

.mode-selector {
    padding-bottom: 24px;
    border-bottom: 1px solid var(--border-light);
}

/* Primary vs secondary button distinction */
.btn-outline {
    font-weight: 500;
}

.btn-primary,
.btn-success,
.btn-danger {
    font-weight: 700;
    letter-spacing: 0.01em;
}

/* ===== 22. Spacing Consistency (8px grid) ===== */

/* Ensure card internal padding is generous */
.learn-card {
    padding: 40px 28px;
}

.write-card {
    padding: 40px 28px;
}

.test-question {
    padding: 24px;
}

.test-setup {
    padding: 40px 32px;
}

/* Consistent gaps */
.flashcard-actions {
    gap: 16px;
    margin-top: 8px;
}

.learn-actions {
    gap: 16px;
    margin-top: 24px;
}

.learn-stats {
    gap: 24px;
    margin-top: 24px;
}

/* ===== 23. Additional Animations ===== */

@keyframes fadeInOutHint {
    0%, 100% {
        opacity: 0.35;
    }
    50% {
        opacity: 0.65;
    }
}

@keyframes starFillGold {
    0% {
        transform: scale(1);
        filter: drop-shadow(0 0 0 rgba(245, 158, 11, 0));
    }
    30% {
        transform: scale(1.4);
        filter: drop-shadow(0 0 12px rgba(245, 158, 11, 0.6));
    }
    50% {
        transform: scale(0.9);
    }
    70% {
        transform: scale(1.15);
    }
    100% {
        transform: scale(1);
        filter: drop-shadow(0 0 8px rgba(245, 158, 11, 0.5));
    }
}

@keyframes leitnerPulse {
    0%, 100% {
        box-shadow: 0 0 0 6px rgba(79, 110, 247, 0.3), 0 4px 12px rgba(0,0,0,0.12);
    }
    50% {
        box-shadow: 0 0 0 10px rgba(79, 110, 247, 0.15), 0 6px 16px rgba(0,0,0,0.15);
    }
}

/* ===== 24. Responsive Enhancements (768px overlay arrows) ===== */
@media (max-width: 768px) {
    /* Streak: compact */
    .streak {
        font-size: 0.85rem;
        padding: 5px 12px;
    }

    /* Nav arrows: overlay on card, semi-transparent */
    .flashcard-container {
        position: relative;
    }

    .nav-arrow {
        position: absolute;
        z-index: 10;
        opacity: 0.7;
        background: rgba(255, 255, 255, 0.85);
        backdrop-filter: blur(6px);
        -webkit-backdrop-filter: blur(6px);
    }

    .nav-arrow:hover {
        opacity: 1;
    }

    .nav-prev {
        left: 4px;
    }

    .nav-next {
        right: 4px;
    }

    .flashcard {
        margin: 0 auto;
    }
}

/* ===== 25. Responsive Enhancements (480px) ===== */
@media (max-width: 480px) {
    /* Mini progress bar hidden */
    .mini-progress {
        display: none;
    }

    /* Leitner boxes: horizontal scroll if needed */
    .leitner-boxes {
        overflow-x: auto;
        flex-wrap: nowrap;
        scrollbar-width: none;
    }

    .leitner-boxes::-webkit-scrollbar {
        display: none;
    }

    .leitner-box {
        min-width: 54px;
        flex-shrink: 0;
    }

    /* Modal: full-screen on mobile */
    .modal {
        padding: 0;
        align-items: stretch;
    }

    .modal-content {
        max-width: 100%;
        max-height: 100vh;
        border-radius: 0;
        padding: 24px 18px;
    }

    /* Nav arrows overlay */
    .nav-arrow {
        position: absolute;
        z-index: 10;
        opacity: 0.65;
        background: rgba(255, 255, 255, 0.9);
        backdrop-filter: blur(4px);
        -webkit-backdrop-filter: blur(4px);
    }

    .nav-prev {
        left: 2px;
    }

    .nav-next {
        right: 2px;
    }
}

/* ===== 26. Responsive - Extra Small (360px) ===== */
@media (max-width: 360px) {
    body {
        padding-top: 52px;
    }

    .top-nav {
        height: 52px;
        padding: 0 10px;
    }

    .nav-logo {
        font-size: 1.2rem;
    }

    .streak {
        padding: 4px 10px;
        font-size: 0.78rem;
        min-height: 44px;
        min-width: 44px;
    }

    .streak::before {
        font-size: 0.85rem;
    }

    .nav-btn {
        width: 44px;
        height: 44px;
        font-size: 1rem;
    }

    .mode-selector {
        margin-bottom: 16px;
        padding-bottom: 16px;
    }

    .mode-btn {
        padding: 7px 10px;
        font-size: 0.78rem;
        border-radius: 36px;
        min-height: 44px;
        min-width: 44px;
    }

    .mode-btn .mode-icon {
        font-size: 1.15rem;
    }

    .flashcard-front,
    .flashcard-back {
        padding: 20px 14px;
    }

    .card-badge {
        font-size: 0.65rem;
        padding: 3px 10px;
        margin-bottom: 12px;
    }

    .card-code {
        font-size: 0.75rem;
    }

    .flip-hint {
        font-size: 0.65rem;
        bottom: 12px;
    }

    .nav-arrow {
        width: 44px;
        height: 44px;
        font-size: 1.1rem;
    }

    .flashcard-container {
        gap: 6px;
    }

    .flashcard-progress {
        font-size: 0.85rem;
        gap: 8px;
        margin-bottom: 14px;
    }

    .match-grid {
        gap: 6px;
    }

    .match-card {
        padding: 10px 6px;
        font-size: 0.82rem;
        min-height: 56px;
    }

    .leitner-boxes {
        padding: 10px 6px;
        gap: 4px;
    }

    .leitner-box {
        padding: 6px 2px;
    }

    .box-label {
        font-size: 0.58rem;
    }

    .box-count {
        font-size: 0.9rem;
    }

    .learn-card,
    .write-card {
        padding: 24px 14px;
    }

    .learn-term,
    .write-term {
        font-size: 1.05rem;
    }

    .test-question {
        padding: 16px 12px;
    }

    .test-setup {
        padding: 24px 16px;
    }

    .btn {
        padding: 9px 14px;
        font-size: 0.82rem;
        min-height: 44px;
    }

    .btn-lg {
        padding: 12px 20px;
        font-size: 0.88rem;
    }

    .modal-content {
        padding: 24px 16px;
        border-radius: var(--radius-sm);
    }

    .flashcard-back .katex,
    .card-formula .katex {
        font-size: 1.35em;
    }

    .learn-card .katex,
    .learn-answer .katex {
        font-size: 1.15em;
    }

    .card-example {
        padding: 14px 12px;
        margin-top: 16px;
    }

    .example-header {
        font-size: 0.75rem;
    }

    .example-question {
        font-size: 0.875rem;
    }

    .example-input {
        padding: 8px 10px;
        font-size: 0.875rem;
        min-height: 44px;
    }

    /* Hide non-essential elements */
    .card-units {
        display: none;
    }

    .write-score {
        display: none;
    }

    .match-best {
        display: none;
    }
}

/* iOS input zoom prevention */
@media (max-width: 768px) {
    input, select, textarea {
        font-size: 16px;
    }
}

/* Dark mode removed — always light theme */