/* ===== CSS Variables & Reset ===== */
:root {
  --bg: #f5f7fa;
  --bg-card: #ffffff;
  --bg-card-hover: #f0f4f8;
  --accent: #1a73e8;
  --accent-light: #4a9af5;
  --accent-dark: #1557b0;
  --text: #1a1a2e;
  --text-dim: #6b7280;
  --text-bright: #0a0a1a;
  --success: #00b894;
  --error: #e74c3c;
  --warning: #fdcb6e;
  --border: #dde1e8;
  --sidebar-width: 280px;
  --radius: 12px;
  --radius-sm: 8px;
}

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

html {
  scroll-behavior: smooth;
  overflow-x: hidden;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.7;
  font-size: 16px;
  overflow-x: hidden;
}

/* ===== Sidebar ===== */
.sidebar {
  position: fixed;
  top: 0;
  left: 0;
  width: var(--sidebar-width);
  height: 100vh;
  background: var(--bg-card);
  border-right: 1px solid var(--border);
  overflow-y: auto;
  z-index: 100;
  transition: transform 0.3s ease;
}

.sidebar-header {
  padding: 24px 20px 16px;
  border-bottom: 1px solid var(--border);
}

.sidebar-header h2 {
  font-size: 20px;
  color: var(--accent-light);
  margin-bottom: 4px;
}

.sidebar-subtitle {
  font-size: 13px;
  color: var(--text-dim);
  margin-bottom: 12px;
}

.progress-container {
  width: 100%;
  height: 6px;
  background: var(--border);
  border-radius: 3px;
  overflow: hidden;
}

.progress-bar {
  height: 100%;
  width: 0%;
  background: linear-gradient(90deg, var(--accent), var(--accent-light));
  border-radius: 3px;
  transition: width 0.5s ease;
}

.progress-text {
  font-size: 12px;
  color: var(--text-dim);
  display: block;
  margin-top: 4px;
}

.nav-list {
  list-style: none;
  padding: 12px 0;
}

.nav-divider {
  padding: 16px 20px 6px;
  font-size: 11px;
  text-transform: uppercase;
  letter-spacing: 1.5px;
  color: var(--text-dim);
  font-weight: 600;
}

.nav-link {
  display: block;
  padding: 10px 20px;
  color: var(--text);
  text-decoration: none;
  font-size: 14px;
  transition: all 0.2s;
  border-left: 3px solid transparent;
}

.nav-link:hover {
  background: rgba(26, 115, 232, 0.1);
  color: var(--accent-light);
}

.nav-link.active {
  background: rgba(26, 115, 232, 0.15);
  color: var(--accent-light);
  border-left-color: var(--accent);
  font-weight: 500;
}

.nav-link.completed {
  color: var(--success);
}

.nav-link.completed::after {
  content: ' \2713';
  font-weight: bold;
}

/* ===== Sidebar Toggle (Mobile) ===== */
.sidebar-toggle {
  display: none;
  position: fixed;
  top: 16px;
  left: 16px;
  z-index: 200;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 10px;
  cursor: pointer;
  flex-direction: column;
  gap: 4px;
  min-width: 44px;
  min-height: 44px;
  align-items: center;
  justify-content: center;
}

.sidebar-toggle span {
  display: block;
  width: 22px;
  height: 2px;
  background: var(--text);
  transition: 0.3s;
}

.overlay {
  display: none;
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: rgba(0,0,0,0.3);
  z-index: 90;
}

.overlay.active {
  display: block;
}

/* ===== Main Content ===== */
.main-content {
  margin-left: var(--sidebar-width);
  padding: 40px 48px;
  max-width: 900px;
}

.content-section {
  display: none;
}

.content-section.active {
  display: block;
  animation: fadeIn 0.3s ease;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(8px); }
  to { opacity: 1; transform: translateY(0); }
}

h1 {
  font-size: 32px;
  font-weight: 700;
  color: var(--text-bright);
  margin-bottom: 24px;
  padding-bottom: 12px;
  border-bottom: 2px solid var(--accent);
}

.section-desc {
  color: var(--text-dim);
  margin-bottom: 24px;
  font-size: 15px;
}

/* ===== Cards ===== */
.card {
  background: var(--bg-card);
  border-radius: var(--radius);
  padding: 24px;
  margin-bottom: 20px;
  border: 1px solid var(--border);
  transition: border-color 0.3s;
}

.card:hover {
  border-color: rgba(26, 115, 232, 0.3);
}

.card h3 {
  font-size: 18px;
  color: var(--accent-light);
  margin-bottom: 16px;
}

.card h4 {
  font-size: 16px;
  color: var(--text-bright);
  margin: 16px 0 8px;
}

.card-body p {
  margin-bottom: 10px;
}

.card-body ul, .card-body ol {
  padding-left: 24px;
  margin-bottom: 12px;
}

.card-body li {
  margin-bottom: 6px;
}

/* ===== Formula Boxes ===== */
.formula-box {
  background: rgba(26, 115, 232, 0.08);
  border: 1px solid rgba(26, 115, 232, 0.2);
  border-radius: var(--radius-sm);
  padding: 16px 20px;
  margin: 12px 0;
  text-align: center;
  font-size: 17px;
}

.formula-box.highlight {
  background: rgba(26, 115, 232, 0.15);
  border-color: var(--accent);
}

/* ===== Example Boxes ===== */
.example-box {
  background: rgba(0, 184, 148, 0.08);
  border-left: 4px solid var(--success);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  padding: 16px 20px;
  margin: 12px 0;
}

/* ===== Buttons ===== */
.btn-primary {
  background: var(--accent);
  color: white;
  border: none;
  padding: 12px 28px;
  border-radius: var(--radius-sm);
  font-size: 15px;
  cursor: pointer;
  transition: all 0.2s;
  font-weight: 500;
}

.btn-primary:hover {
  background: var(--accent-dark);
  transform: translateY(-1px);
}

.btn-mark-read {
  background: transparent;
  color: var(--accent-light);
  border: 1px solid var(--accent);
  padding: 10px 24px;
  border-radius: var(--radius-sm);
  font-size: 14px;
  cursor: pointer;
  margin-top: 16px;
  transition: all 0.2s;
}

.btn-mark-read:hover {
  background: var(--accent);
  color: white;
}

.btn-mark-read.completed {
  background: var(--success);
  border-color: var(--success);
  color: white;
}

/* ===== Canvas ===== */
.canvas-container {
  background: var(--bg-card);
  border-radius: var(--radius);
  border: 1px solid var(--border);
  padding: 16px;
  margin-bottom: 16px;
  overflow: hidden;
}

.canvas-container canvas {
  width: 100%;
  height: auto;
  display: block;
  cursor: grab;
}

.canvas-container canvas:active {
  cursor: grabbing;
}

.canvas-info {
  background: var(--bg-card);
  border-radius: var(--radius);
  border: 1px solid var(--border);
  padding: 16px 20px;
  margin-bottom: 16px;
}

.info-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
  gap: 10px;
}

.info-item {
  font-size: 14px;
}

.info-label {
  color: var(--text-dim);
  margin-right: 6px;
}

.canvas-controls {
  display: flex;
  flex-wrap: wrap;
  gap: 16px;
  padding: 12px;
  background: var(--bg-card);
  border-radius: var(--radius);
  border: 1px solid var(--border);
}

.canvas-controls label {
  display: flex;
  align-items: center;
  gap: 6px;
  font-size: 14px;
  color: var(--text);
  cursor: pointer;
}

.canvas-controls input[type="checkbox"] {
  accent-color: var(--accent);
  width: 16px;
  height: 16px;
}

/* ===== Problems ===== */
.problem-filters {
  display: flex;
  gap: 8px;
  margin-bottom: 24px;
  flex-wrap: wrap;
}

.filter-btn {
  padding: 8px 16px;
  border-radius: 20px;
  border: 1px solid var(--border);
  background: transparent;
  color: var(--text);
  cursor: pointer;
  font-size: 13px;
  transition: all 0.2s;
}

.filter-btn:hover, .filter-btn.active {
  background: var(--accent);
  border-color: var(--accent);
  color: white;
}

.problem-card {
  background: var(--bg-card);
  border-radius: var(--radius);
  border: 1px solid var(--border);
  padding: 24px;
  margin-bottom: 16px;
}

.problem-card .problem-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 12px;
}

.problem-card .problem-number {
  font-size: 13px;
  color: var(--accent-light);
  font-weight: 600;
}

.problem-card .problem-tag {
  font-size: 11px;
  padding: 3px 10px;
  border-radius: 12px;
  background: rgba(26, 115, 232, 0.15);
  color: var(--accent-light);
}

.problem-card .problem-text {
  font-size: 15px;
  margin-bottom: 16px;
  line-height: 1.8;
}

.problem-card .problem-answer-row {
  display: flex;
  align-items: center;
  gap: 12px;
  margin-bottom: 12px;
}

.problem-card .answer-input {
  width: 120px;
}

.problem-card .btn-check-answer {
  background: var(--accent);
  color: white;
  border: none;
  padding: 8px 20px;
  border-radius: var(--radius-sm);
  cursor: pointer;
  font-size: 14px;
  transition: 0.2s;
}

.problem-card .btn-check-answer:hover {
  background: var(--accent-dark);
}

.problem-card .problem-result {
  font-size: 15px;
  font-weight: 700;
  min-height: 20px;
  padding: 12px 16px;
  border-radius: 8px;
  margin-top: 10px;
  transition: all 0.3s ease;
}

.problem-card .problem-result.correct {
  color: #00b894;
  background: rgba(0, 184, 148, 0.15);
  border: 2px solid rgba(0, 184, 148, 0.5);
  box-shadow: 0 0 15px rgba(0, 184, 148, 0.2);
  animation: correctPulse 0.5s ease;
}

.problem-card .problem-result.wrong {
  color: #e17055;
  background: rgba(225, 112, 85, 0.15);
  border: 2px solid rgba(225, 112, 85, 0.5);
  box-shadow: 0 0 15px rgba(225, 112, 85, 0.2);
  animation: wrongShake 0.4s ease;
}

@keyframes correctPulse {
  0% { transform: scale(0.95); opacity: 0; }
  50% { transform: scale(1.02); }
  100% { transform: scale(1); opacity: 1; }
}

@keyframes wrongShake {
  0%, 100% { transform: translateX(0); }
  20% { transform: translateX(-6px); }
  40% { transform: translateX(6px); }
  60% { transform: translateX(-4px); }
  80% { transform: translateX(4px); }
}

.problem-card .btn-show-solution {
  background: transparent;
  color: var(--accent-light);
  border: 1px solid var(--border);
  padding: 8px 16px;
  border-radius: var(--radius-sm);
  cursor: pointer;
  font-size: 13px;
  transition: 0.2s;
}

.problem-card .btn-show-solution:hover {
  border-color: var(--accent);
}

.problem-card .solution {
  display: none;
  margin-top: 16px;
  padding: 16px;
  background: rgba(26, 115, 232, 0.06);
  border-radius: var(--radius-sm);
  border-left: 3px solid var(--accent);
  line-height: 1.9;
  font-size: 14px;
}

.problem-card .solution.visible {
  display: block;
}

.problem-card .solution p {
  margin-bottom: 8px;
}

/* ===== Trainers ===== */
.trainer-container {
  max-width: 600px;
}

.trainer-score {
  background: var(--bg-card);
  border-radius: var(--radius-sm);
  padding: 12px 20px;
  margin-bottom: 20px;
  font-size: 16px;
  border: 1px solid var(--border);
}

.trainer-question {
  margin-bottom: 20px;
}

.options-grid {
  display: grid;
  gap: 10px;
  margin-top: 16px;
}

.option-btn {
  display: block;
  width: 100%;
  text-align: left;
  padding: 14px 18px;
  background: rgba(26, 115, 232, 0.05);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  color: var(--text);
  cursor: pointer;
  font-size: 15px;
  transition: all 0.2s;
}

.option-btn:hover {
  border-color: var(--accent);
  background: rgba(26, 115, 232, 0.1);
}

.option-btn.correct {
  border-color: var(--success);
  background: rgba(0, 184, 148, 0.15);
  color: var(--success);
}

.option-btn.wrong {
  border-color: var(--error);
  background: rgba(231, 76, 60, 0.15);
  color: var(--error);
}

.option-btn.disabled {
  pointer-events: none;
  opacity: 0.7;
}

.feedback {
  margin-top: 12px;
  padding: 12px;
  border-radius: var(--radius-sm);
  font-size: 14px;
  min-height: 20px;
}

.feedback.correct {
  background: rgba(0, 184, 148, 0.1);
  color: var(--success);
}

.feedback.wrong {
  background: rgba(231, 76, 60, 0.1);
  color: var(--error);
}

/* ===== Answer Input ===== */
.answer-input {
  background: var(--bg);
  border: 1px solid var(--border);
  color: var(--text-bright);
  padding: 10px 16px;
  border-radius: var(--radius-sm);
  font-size: 16px;
  width: 160px;
  transition: border-color 0.2s;
}

.answer-input:focus {
  outline: none;
  border-color: var(--accent);
}

.answer-input-group {
  display: flex;
  align-items: center;
  gap: 8px;
  margin: 12px 0;
}

.input-suffix {
  color: var(--text-dim);
  font-size: 16px;
}

/* ===== Exam Mode ===== */
.exam-header {
  display: flex;
  align-items: center;
  gap: 24px;
  margin-bottom: 24px;
  padding: 16px;
  background: var(--bg-card);
  border-radius: var(--radius);
  border: 1px solid var(--border);
}

.exam-timer {
  font-size: 32px;
  font-weight: 700;
  color: var(--accent-light);
  font-variant-numeric: tabular-nums;
}

.exam-timer.warning {
  color: var(--warning);
}

.exam-timer.danger {
  color: var(--error);
  animation: pulse 1s infinite;
}

@keyframes pulse {
  0%, 100% { opacity: 1; }
  50% { opacity: 0.5; }
}

.exam-progress {
  color: var(--text-dim);
  font-size: 15px;
}

.exam-problem {
  padding: 24px;
}

.exam-results {
  padding: 24px;
}

.exam-results h2 {
  margin-bottom: 16px;
  color: var(--text-bright);
}

.exam-result-item {
  padding: 12px 16px;
  background: var(--bg-card);
  border-radius: var(--radius-sm);
  margin-bottom: 8px;
  border: 1px solid var(--border);
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.exam-result-item.correct {
  border-left: 3px solid var(--success);
}

.exam-result-item.wrong {
  border-left: 3px solid var(--error);
}

.exam-score {
  font-size: 24px;
  font-weight: 700;
  color: var(--accent-light);
  margin: 24px 0;
}

/* ===== Cheatsheet ===== */
.cheat-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 16px;
}

.cheat-card {
  padding: 20px;
}

.cheat-card p {
  margin-bottom: 6px;
  font-size: 14px;
}

/* ===== Responsive ===== */
@media (max-width: 900px) {
  .sidebar {
    transform: translateX(-100%);
  }

  .sidebar.open {
    transform: translateX(0);
    z-index: 200;
    box-shadow: 4px 0 20px rgba(0, 0, 0, 0.15);
  }

  .sidebar-toggle {
    display: flex;
  }

  .main-content {
    margin-left: 0;
    padding: 24px 16px;
    padding-top: 60px;
  }

  /* Tables: horizontal scroll */
  .styled-table {
    display: block;
    overflow-x: auto;
    -webkit-overflow-scrolling: touch;
  }

  h1 {
    font-size: 24px;
  }

  .exam-header {
    flex-direction: column;
    align-items: flex-start;
    gap: 12px;
  }

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

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

  .canvas-controls {
    flex-direction: column;
    gap: 10px;
  }
}

@media (max-width: 500px) {
  .main-content {
    padding: 16px 12px;
    padding-top: 56px;
  }

  .card {
    padding: 16px;
  }

  .formula-box {
    padding: 12px;
    font-size: 15px;
    overflow-x: auto;
    -webkit-overflow-scrolling: touch;
  }

  .problem-filters {
    gap: 6px;
  }

  .filter-btn {
    padding: 6px 12px;
    font-size: 12px;
    min-height: 44px;
  }

  /* Touch targets */
  .btn, .nav-link, button {
    min-height: 44px;
  }

  .nav-link {
    padding: 12px 14px;
  }

  /* Trainer inputs full width */
  .trainer-input-row {
    flex-direction: column;
    align-items: stretch;
  }

  .trainer-input {
    width: 100%;
  }
}

/* iOS input zoom prevention */
@media (max-width: 900px) {
  input, select, textarea {
    font-size: 16px;
  }
}

/* ===== Scrollbar ===== */
::-webkit-scrollbar {
  width: 8px;
}

::-webkit-scrollbar-track {
  background: var(--bg);
}

::-webkit-scrollbar-thumb {
  background: var(--border);
  border-radius: 4px;
}

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

/* ===== Algorithm Steps ===== */
.algorithm-card {
  border-color: rgba(26, 115, 232, 0.4);
}

.algorithm-steps {
  display: flex;
  flex-direction: column;
  gap: 10px;
}

.algorithm-step {
  display: flex;
  align-items: flex-start;
  gap: 14px;
  padding: 12px 16px;
  background: rgba(26, 115, 232, 0.06);
  border-radius: var(--radius-sm);
  border-left: 3px solid var(--accent);
  transition: background 0.2s;
}

.algorithm-step:hover {
  background: rgba(26, 115, 232, 0.12);
}

.step-number {
  display: flex;
  align-items: center;
  justify-content: center;
  min-width: 32px;
  height: 32px;
  border-radius: 50%;
  background: var(--accent);
  color: white;
  font-weight: 700;
  font-size: 15px;
  flex-shrink: 0;
}

.step-content {
  font-size: 14px;
  line-height: 1.6;
  padding-top: 4px;
}

/* ===== Mistakes ===== */
.mistakes-card {
  border-color: rgba(231, 76, 60, 0.4);
}

.mistake-item {
  margin-bottom: 20px;
  padding: 16px;
  background: rgba(231, 76, 60, 0.05);
  border-radius: var(--radius-sm);
  border-left: 4px solid var(--error);
}

.mistake-item:last-child {
  margin-bottom: 0;
}

.mistake-header {
  display: flex;
  align-items: center;
  gap: 12px;
  margin-bottom: 8px;
  font-weight: 600;
  font-size: 15px;
}

.mistake-badge {
  display: inline-block;
  padding: 2px 10px;
  background: var(--error);
  color: white;
  border-radius: 12px;
  font-size: 12px;
  font-weight: 700;
  white-space: nowrap;
}

/* ===== Remember / Zapomni ===== */
.remember-card {
  border-color: rgba(253, 203, 110, 0.5);
  background: linear-gradient(135deg, rgba(253, 203, 110, 0.08), rgba(26, 115, 232, 0.06));
}

.remember-card h3 {
  color: var(--warning);
}

.remember-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: 12px;
}

.remember-item {
  display: flex;
  align-items: center;
  gap: 12px;
  padding: 14px 16px;
  background: rgba(253, 203, 110, 0.1);
  border: 1px solid rgba(253, 203, 110, 0.25);
  border-radius: var(--radius-sm);
  transition: transform 0.2s, border-color 0.2s;
}

.remember-item:hover {
  border-color: var(--warning);
  transform: translateY(-2px);
}

.remember-number {
  display: flex;
  align-items: center;
  justify-content: center;
  min-width: 34px;
  height: 34px;
  border-radius: 50%;
  background: var(--warning);
  color: #ffffff;
  font-weight: 800;
  font-size: 16px;
  flex-shrink: 0;
}

.remember-text {
  font-size: 14px;
  line-height: 1.5;
  color: var(--text-bright);
}

/* ===== Problems Counter ===== */
.problems-counter-row {
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-wrap: wrap;
  gap: 12px;
  margin-bottom: 24px;
}

.problems-counter-row .problem-filters {
  margin-bottom: 0;
}

.problems-counter {
  font-size: 15px;
  font-weight: 600;
  color: var(--accent-light);
  padding: 8px 18px;
  background: rgba(26, 115, 232, 0.12);
  border-radius: 20px;
  border: 1px solid rgba(26, 115, 232, 0.25);
}

/* ===== Problem Solved State ===== */
.problem-card.problem-solved {
  border-color: rgba(0, 184, 148, 0.3);
}

.problem-solved-badge {
  font-size: 11px;
  padding: 2px 10px;
  border-radius: 12px;
  background: rgba(0, 184, 148, 0.2);
  color: var(--success);
  font-weight: 700;
}

/* ===== Hints ===== */
.problem-actions {
  display: flex;
  gap: 10px;
  margin-bottom: 8px;
}

.btn-show-hint {
  background: transparent;
  color: var(--warning);
  border: 1px solid rgba(253, 203, 110, 0.4);
  padding: 8px 16px;
  border-radius: var(--radius-sm);
  cursor: pointer;
  font-size: 13px;
  transition: 0.2s;
}

.btn-show-hint:hover {
  border-color: var(--warning);
  background: rgba(253, 203, 110, 0.1);
}

.btn-show-hint.disabled {
  opacity: 0.5;
  cursor: default;
}

.hints-container {
  margin-bottom: 8px;
}

.hint-item {
  padding: 10px 14px;
  margin-top: 6px;
  border-radius: var(--radius-sm);
  font-size: 14px;
  line-height: 1.6;
  animation: fadeIn 0.3s ease;
}

.hint-item.hint-level-1 {
  background: rgba(253, 203, 110, 0.08);
  border-left: 3px solid rgba(253, 203, 110, 0.5);
  color: var(--warning);
}

.hint-item.hint-level-2 {
  background: rgba(26, 115, 232, 0.08);
  border-left: 3px solid rgba(26, 115, 232, 0.5);
  color: var(--accent-light);
}

.hint-item.hint-level-3 {
  background: rgba(0, 184, 148, 0.08);
  border-left: 3px solid rgba(0, 184, 148, 0.5);
  color: var(--success);
}

/* ===== KaTeX overrides ===== */
.katex {
  font-size: 1.05em;
}

.katex-display {
  margin: 0.5em 0;
}