body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', sans-serif;
  line-height: 1.6;
  color: #1e293b;
  max-width: 900px;
  margin: 0 auto;
  padding: 2rem;
  background: #f8fafc;
}

.section-controls,
.mcq-controls {
  display: flex;
  flex-wrap: wrap;
  gap: 0.75rem;
  align-items: center;
  margin: 1.2rem 0 1.5rem;
}

.section-controls button:focus-visible,
.mcq-controls button:focus-visible {
  outline: 2px solid #2563eb;
  outline-offset: 2px;
}

.section-controls button,
.mcq-controls button {
  border: 1px solid #cbd5e1;
  background: #ffffff;
  color: #334155;
  font-size: 0.95rem;
  font-weight: 700;
  padding: 0.65rem 1.2rem;
  border-radius: 999px;
  cursor: pointer;
  transition: all 0.2s ease;
  box-shadow: 0 2px 8px rgba(2, 6, 23, 0.06);
}

.section-controls button:hover,
.mcq-controls button:hover {
  background: #f1f5f9;
  border-color: #94a3b8;
  box-shadow: 0 4px 12px rgba(2, 6, 23, 0.1);
  transform: translateY(-1px);
}

.section-controls button.active,
.mcq-controls button.active {
  background: linear-gradient(135deg, #2563eb, #1d4ed8);
  color: #ffffff;
  border-color: #2563eb;
  box-shadow: 0 4px 12px rgba(37, 99, 235, 0.3);
}

.toc h2 {
  margin-top: 0;
  font-size: 1.1rem;
  color: #475569;
  border: none;
}

.toc ol {
  columns: 2;
  column-gap: 2rem;
  padding-left: 1.25rem;
}

.toc a {
  color: #2563eb;
  text-decoration: none;
}

.toc a:hover {
  text-decoration: underline;
}

.toc a:focus-visible {
  outline: 2px solid #2563eb;
  outline-offset: 2px;
  border-radius: 2px;
}

.unit-header {
  background: linear-gradient(135deg, #0f172a 0%, #1e3a5f 100%);
  color: #f8fafc;
  padding: 1.75rem 2rem;
  border-radius: 12px;
  margin: 3rem 0 1.5rem;
}

.unit-header h2 {
  margin: 0;
  font-size: 1.5rem;
  border: none;
  color: #f8fafc;
}

.unit-code {
  font-size: 0.85rem;
  opacity: 0.75;
  font-weight: 400;
  letter-spacing: 0.05em;
}

/* Collapsible units */
.unit-details {
  margin: 3rem 0 1.5rem;
  border: none;
  background: none;
}

.unit-details .unit-header {
  cursor: pointer;
  user-select: none;
  display: flex;
  align-items: center;
  gap: 0.5rem;
  margin: 0;
}

.unit-details .unit-header h2 {
  flex: 1;
}

.unit-stats {
  font-size: 0.8rem;
  opacity: 0.8;
  white-space: nowrap;
  flex-shrink: 0;
}

.unit-details .unit-header::-webkit-details-marker {
  display: none;
}

.unit-details .unit-header::marker {
  display: none;
  content: '';
}

.collapse-icon {
  font-size: 0.8rem;
  color: rgba(255,255,255,0.6);
  transition: transform 0.2s;
  flex-shrink: 0;
}

.unit-details[open] .collapse-icon {
  transform: rotate(0deg);
}

.unit-details:not([open]) .collapse-icon {
  transform: rotate(-90deg);
}

.unit-body {
  padding: 0;
}

.unit-details:not([open]) .unit-body {
  display: none;
}

.unit-stats {
  margin-top: 0.5rem;
  font-size: 0.9rem;
  opacity: 0.85;
}

.section-header {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  margin: 2.2rem 0 1.4rem;
  padding-bottom: 0.75rem;
  border-bottom: 2px solid #e2e8f0;
}

.section-header h3 {
  margin: 0;
  font-size: 1.2rem;
  color: #1e293b;
  font-weight: 700;
}

.section-badge {
  background: linear-gradient(135deg, #f0f9ff, #e0f2fe);
  color: #0369a1;
  font-size: 0.75rem;
  font-weight: 700;
  padding: 0.35rem 0.75rem;
  border-radius: 999px;
  letter-spacing: 0.05em;
  box-shadow: 0 2px 4px rgba(3, 105, 161, 0.1);
}

.question {
  border-radius: 8px;
  padding: 0.8rem 1.1rem;
  margin: 0.6rem 0;
  box-shadow: 0 2px 6px rgba(0, 0, 0, 0.06);
  background: #ffffff;
  border: 1px solid #f1f5f9;
  transition: all 0.2s ease;
  position: relative;
}

.question:hover {
  box-shadow: 0 6px 16px rgba(0, 0, 0, 0.1);
  transform: translateY(-2px);
  border-color: #e2e8f0;
}

.q-num {
  font-size: 0.68rem;
  font-weight: 700;
  color: #94a3b8;
  letter-spacing: 0.08em;
  margin-bottom: 0.2rem;
}

.q-text {
  font-size: 0.86rem;
  font-weight: 500;
  color: #1e293b;
  margin-bottom: 0.5rem;
}

.options {
  display: flex;
  flex-direction: column;
  gap: 0.25rem;
}

.opt {
  padding: 0.3rem 0.6rem;
  border-radius: 5px;
  font-size: 0.83rem;
  color: #334155;
  background: #f8fafc;
}

.opt-letter {
  font-weight: 700;
  color: #64748b;
  margin-right: 0.35rem;
}

.answer-reveal {
  margin-top: 0.35rem;
}

.answer-reveal summary {
  cursor: pointer;
  list-style: none;
  font-size: 0.75rem;
  color: #64748b;
  padding: 0.2rem 0;
}

.answer-reveal summary::before {
  content: "▸ Reveal answer";
}

.answer-reveal[open] summary::before {
  content: "▾ Hide answer";
}

.answer-reveal summary::-webkit-details-marker {
  display: none;
}

.answer {
  padding: 0.5rem 0.8rem;
  border-radius: 5px;
  font-size: 0.83rem;
  color: #1e293b;
  margin-top: 0.2rem;
  background: #f1f5f9;
}

.mcq-controls {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  position: sticky;
  top: 0;
  z-index: 1000;
  background: rgba(248, 250, 252, 0.95);
  backdrop-filter: blur(8px);
  -webkit-backdrop-filter: blur(8px);
  padding: 1rem 0;
  border-bottom: 1px solid #e2e8f0;
  margin: 0 0 1.5rem 0 !important;
}

.progress-bar-container {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  flex: 1;
  min-width: 150px;
  max-width: 320px;
  opacity: 0;
  transform: translateY(4px);
  transition: opacity 0.35s ease, transform 0.35s ease;
  pointer-events: none;
}

.progress-bar-container.active {
  opacity: 1;
  transform: translateY(0);
  pointer-events: auto;
}

.progress-bar-track {
  flex: 1;
  height: 8px;
  background: rgba(203, 213, 225, 0.35);
  border-radius: 999px;
  overflow: hidden;
  position: relative;
  box-shadow: inset 0 1px 3px rgba(0,0,0,0.06);
}

.progress-bar-fill {
  height: 100%;
  width: 0%;
  border-radius: 999px;
  transition: width 0.5s cubic-bezier(0.4, 0, 0.2, 1), box-shadow 0.4s ease;
  position: relative;
}

.progress-bar-fill::after {
  content: '';
  position: absolute;
  top: 0; left: 0; right: 0; bottom: 0;
  background: linear-gradient(90deg, transparent 20%, rgba(255,255,255,0.35) 50%, transparent 80%);
  animation: progress-shine 3s ease-in-out infinite;
  border-radius: 999px;
}

@keyframes progress-shine {
  0% { transform: translateX(-100%); }
  100% { transform: translateX(300%); }
}

.progress-label {
  font-size: 0.72rem;
  font-weight: 700;
  color: #475569;
  white-space: nowrap;
  letter-spacing: -0.01em;
  min-width: 88px;
  text-align: right;
  font-variant-numeric: tabular-nums;
  transition: color 0.3s ease;
}

/* Redesigned pill switches */
.switch {
  display: inline-flex;
  cursor: pointer;
  padding: 0;
  margin: 0;
  gap: 0;
}

.switch input { display: none; }

.switch-track {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  height: 32px;
  padding: 0 4px 0 12px;
  border-radius: 999px;
  background: #e6eef8;
  transition: background .18s, box-shadow .18s;
  position: relative;
  box-shadow: inset 0 1px 2px rgba(2, 6, 23, 0.04);
}

.switch input:checked + .switch-track {
  background: linear-gradient(90deg, #3b82f6, #06b6d4);
  box-shadow: inset 0 1px 3px rgba(2, 6, 23, 0.08);
}

.switch-label {
  font-weight: 600;
  font-size: 0.82rem;
  color: #475569;
  white-space: nowrap;
  transition: color .18s;
  line-height: 1;
}

.switch input:checked + .switch-track .switch-label {
  color: #fff;
}

.switch-knob {
  width: 22px;
  height: 22px;
  background: #fff;
  border-radius: 50%;
  box-shadow: 0 2px 6px rgba(2, 6, 23, 0.14);
  transition: transform .18s;
  flex-shrink: 0;
}

/* TTS pill group — unified pill with toggle + dots */
.pill-group {
  display: inline-flex;
  align-items: center;
  border-radius: 999px;
  overflow: hidden;
  background: #e6eef8;
  transition: background .18s;
  box-shadow: inset 0 1px 2px rgba(2, 6, 23, 0.04);
  height: 32px;
}

.pill-group.active {
  background: linear-gradient(90deg, #3b82f6, #06b6d4);
}

.pill-group .switch-track {
  height: auto;
  padding: 0 6px 0 12px;
  background: transparent;
  box-shadow: none;
}

.pill-group .switch input:checked + .switch-track {
  background: transparent;
  box-shadow: none;
}

.pill-group .switch .switch-label {
  color: #475569;
}

.pill-group.active .switch .switch-label {
  color: #fff;
}

.pill-group .switch-knob {
  width: 20px;
  height: 20px;
}

.pill-divider {
  width: 1px;
  height: 18px;
  background: #cbd5e1;
  flex-shrink: 0;
  transition: background .18s;
}

.pill-group.active .pill-divider {
  background: rgba(255,255,255,0.35);
}

.pill-dots {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 28px;
  height: 100%;
  border: none;
  background: transparent;
  cursor: pointer;
  color: #94a3b8;
  font-size: 1.15rem;
  padding: 0;
  line-height: 1;
  transition: color .18s;
  flex-shrink: 0;
}

.pill-dots:hover {
  color: #475569;
}

.pill-group.active .pill-dots {
  color: #1e293b;
}

.pill-group.active .pill-dots:hover {
  color: #0f172a;
}

/* TTS settings dropdown */
.tts-settings { position: relative; }

.tts-settings-dropdown {
  position: absolute;
  right: 0;
  top: calc(100% + 14px);
  background: #fff;
  border: 1px solid #e2e8f0;
  border-radius: 10px;
  box-shadow: 0 8px 24px rgba(2, 6, 23, 0.12);
  padding: 0.75rem;
  z-index: 1100;
  min-width: 220px;
  display: flex;
  flex-direction: column;
  gap: 0.6rem;
}

.tts-settings-dropdown[hidden] {
  display: none;
}

.tts-setting-row {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  font-size: 0.9rem;
  color: #334155;
  cursor: pointer;
}

.tts-auto-row {
  gap: 0.4rem;
  padding-bottom: 0.25rem;
  border-bottom: 1px solid #f1f5f9;
  margin-bottom: 0.1rem;
}

.tts-auto-row input[type="checkbox"] {
  accent-color: #3b82f6;
  margin: 0;
  width: 16px;
  height: 16px;
}

.tts-auto-row span {
  font-weight: 600;
  font-size: 0.85rem;
  color: #1e293b;
  min-width: 0;
}

#tts-repeat-row.disabled {
  opacity: 0.4;
  pointer-events: none;
}

.tts-setting-row span:first-child {
  min-width: 48px;
  font-weight: 600;
}

.tts-setting-row input[type="range"] {
  flex: 1;
  accent-color: #3b82f6;
}

.tts-rate-val {
  min-width: 32px;
  text-align: center;
  font-weight: 600;
  color: #2563eb;
  font-size: 0.85rem;
}

.tts-setting-row select {
  flex: 1;
  border: 1px solid #cbd5e1;
  border-radius: 6px;
  padding: 0.3rem 0.4rem;
  font-size: 0.85rem;
  color: #334155;
  background: #fff;
}

.tts-setting-row select:focus-visible {
  outline: 2px solid #2563eb;
  outline-offset: 1px;
}

.tts-scroll-inline {
  display: inline-flex;
  align-items: center;
  gap: 0.3rem;
  font-size: 0.85rem;
  color: #64748b;
  cursor: pointer;
  white-space: nowrap;
  margin-left: auto;
}

.tts-scroll-inline input[type="checkbox"] {
  accent-color: #3b82f6;
  width: 14px;
  height: 14px;
  cursor: pointer;
}

.tts-dropdown-header {
  font-weight: 700;
  font-size: 0.85rem;
  color: #1e293b;
  padding-bottom: 0.4rem;
  border-bottom: 1px solid #e2e8f0;
  margin-bottom: 0.2rem;
}

.tts-dropdown-done {
  width: 100%;
  margin-top: 0.3rem;
  border: none;
  background: linear-gradient(90deg, #3b82f6, #2563eb);
  color: #fff;
  font-weight: 700;
  font-size: 0.85rem;
  padding: 0.5rem;
  border-radius: 8px;
  cursor: pointer;
  transition: opacity 0.15s;
}

.tts-dropdown-done:hover {
  opacity: 0.9;
}

.tts-dropdown-done:focus-visible {
  outline: 2px solid #2563eb;
  outline-offset: 2px;
}

.tts-test-btn {
  width: 100%;
  margin-top: 0.3rem;
  border: 1px solid #cbd5e1;
  background: #fff;
  color: #334155;
  font-weight: 600;
  font-size: 0.82rem;
  padding: 0.45rem;
  border-radius: 8px;
  cursor: pointer;
  transition: background 0.15s;
}

.tts-test-btn:hover {
  background: #f1f5f9;
}

.q-start-btn {
  position: absolute;
  right: 0.75rem;
  top: 50%;
  transform: translateY(-50%);
  border: 1px solid #059669;
  background: #ffffff;
  color: #059669;
  font-size: 0.75rem;
  width: 30px;
  height: 30px;
  border-radius: 50%;
  cursor: pointer;
  transition: all 0.2s ease;
  box-shadow: 0 2px 6px rgba(2, 6, 23, 0.08);
  line-height: 1;
  display: none;
  align-items: center;
  justify-content: center;
  z-index: 2;
}

.question:hover .q-start-btn {
  display: flex;
}

.q-start-btn.playing {
  display: flex;
  background: #ef4444;
  color: #fff;
  border-color: #ef4444;
  animation: q-start-pulse 1.5s ease-in-out infinite;
}

.q-start-btn.playing:hover {
  background: #dc2626;
  border-color: #dc2626;
  transform: translateY(-50%) scale(1.1);
}

.q-start-btn:hover {
  background: #ecfdf5;
  border-color: #047857;
  color: #047857;
  box-shadow: 0 4px 12px rgba(5, 150, 105, 0.25);
  transform: translateY(-50%) scale(1.1);
}

.q-start-btn:focus-visible {
  outline: 2px solid #059669;
  outline-offset: 2px;
}

@keyframes q-start-pulse {
  0%, 100% { box-shadow: 0 0 0 0 rgba(239, 68, 68, 0.4); }
  50% { box-shadow: 0 0 0 6px rgba(239, 68, 68, 0); }
}

.question.tts-playing {
  box-shadow: 0 0 0 3px #3b82f6, 0 6px 16px rgba(0, 0, 0, 0.1);
  position: relative;
}

body.tts-auto-scrolling {
  cursor: none;
}

.section-label {
  font-size: 0.85rem;
  color: #64748b;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  display: block;
  margin-bottom: 0.5rem;
  width: 100%;
}

.hide-answers .opt.correct {
  background: #f8fafc !important;
  border-left: none !important;
}

.hide-answers .opt.correct .checkmark {
  display: none !important;
}

hr.unit-divider {
  border: none;
  height: 1px;
  background: linear-gradient(to right, transparent, #cbd5e1, transparent);
  margin: 3rem 0;
}

.practical-section {
  margin: 2.5rem 0;
}

.dataset-links {
  display: flex;
  flex-wrap: wrap;
  gap: 0.75rem;
  margin-bottom: 1.5rem;
  background: #f8fafc;
  border: 1px solid #e2e8f0;
  border-radius: 10px;
  padding: 1rem 1.25rem;
}

.download-link {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  padding: 0.5rem 1rem;
  background: #fff;
  border: 1px solid #e2e8f0;
  border-radius: 8px;
  color: #1e293b;
  font-size: 0.85rem;
  font-weight: 600;
  text-decoration: none;
  transition: all 0.15s;
  box-shadow: 0 1px 3px rgba(0,0,0,0.04);
}

.download-link::before {
  content: '\21E9';
  font-size: 1rem;
  color: #3b82f6;
}

.download-link:hover {
  background: #f1f5f9;
  border-color: #94a3b8;
  box-shadow: 0 2px 6px rgba(0,0,0,0.08);
}

details.practical-question {
  background: #ffffff;
  border: 1px solid #eef2ff;
  border-radius: 10px;
  padding: 0;
  margin: 1.1rem 0;
  box-shadow: 0 6px 18px rgba(15, 23, 42, 0.04);
  overflow: hidden;
}

details.practical-question>summary {
  list-style: none;
  cursor: pointer;
  padding: 0.9rem 1.25rem;
  font-size: 1.2rem;
  font-weight: 700;
  background: linear-gradient(90deg, #0b4f7a, #1b6a9a);
  color: #ffffff;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  border-radius: 6px 6px 0 0;
}

details.practical-question[open]>summary {
  border-bottom: 1px solid #e6eef8;
}

.practical-body {
  padding: 1rem 1.25rem 1.25rem 1.25rem;
  color: #0f1724;
}

.task-desc {
  margin: 0.45rem 0 0.9rem 0.25rem;
  color: #0f1724;
  font-size: 0.98rem;
  line-height: 1.45;
}

.task-desc ul {
  margin: 0;
  padding-left: 1.05rem;
}

.task-desc li {
  margin: 0.35rem 0;
}

.task-list {
  margin: 0.4rem 0 0.9rem 0;
  padding-left: 1rem;
}

details.solution {
  margin-top: 0.75rem;
}

details.solution summary {
  cursor: pointer;
  list-style: none;
  background: #111827;
  color: #f8fafc;
  padding: 0.6rem 0.8rem;
  border-radius: 8px;
  font-weight: 700;
  margin-bottom: 0.5rem;
}

.practical-question {
  background: #ffffff;
  border: 1px solid #eef2ff;
  border-radius: 10px;
  padding: 1.25rem 1.5rem;
  margin: 1.1rem 0;
  box-shadow: 0 6px 18px rgba(15, 23, 42, 0.04);
}

.practical-task h4 {
  margin: 0 0 0.5rem 0;
  font-size: 1rem;
  color: #0f172a;
  font-weight: 700;
}

.colab-code {
  position: relative;
  background: #0f1724;
  color: #e6eef8;
  padding: 1.4rem 1rem 1rem;
  border-radius: 12px;
  overflow-x: auto;
  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, 'Roboto Mono', 'Courier New', monospace;
  font-size: 0.92rem;
  line-height: 1.5;
  margin: 0.5rem 0 0.75rem 0;
  border: 1px solid #1f2937;
  box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.04), 0 1px 3px rgba(15, 23, 42, 0.12);
}

.colab-code::before {
  content: "Code";
  position: absolute;
  top: 0.75rem;
  left: 1rem;
  font-size: 0.72rem;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: #94a3b8;
  background: rgba(15, 23, 36, 0.8);
  padding: 0.16rem 0.5rem;
  border-radius: 999px;
}

.copy-btn {
  position: absolute;
  top: 0.75rem;
  right: 0.75rem;
  border: none;
  background: rgba(255, 255, 255, 0.08);
  color: #cbd5e1;
  padding: 0.4rem 0.7rem;
  border-radius: 999px;
  cursor: pointer;
  font-size: 0.78rem;
  transition: background .15s, color .15s, transform .15s;
}

.copy-btn:hover {
  background: rgba(255, 255, 255, 0.16);
  color: #fff;
}

.copy-btn:active {
  transform: translateY(1px);
}

.copy-btn:focus-visible {
  outline: 2px solid #fff;
  outline-offset: 2px;
}

.colab-code code {
  white-space: pre;
  display: block;
  margin-top: 1.2rem;
}

details.solution summary {
  cursor: pointer;
  list-style: none;
  background: #111827;
  color: #f8fafc;
  padding: 0.6rem 0.8rem;
  border-radius: 8px;
  font-weight: 700;
  margin-bottom: 0.5rem;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 0.75rem;
}

details.solution summary::before {
  content: "▸";
  margin-right: 0.5rem;
  color: #cbd5e1;
  font-size: 0.95rem;
}

details.solution[open] summary::before {
  content: "▾";
}

.output-images {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
  gap: 0.75rem;
  margin-top: 0.8rem;
}

.output-image {
  width: 100%;
  max-height: 230px;
  object-fit: contain;
  border: 1px solid #e2e8f0;
  border-radius: 10px;
  background: #f8fafc;
  box-shadow: inset 0 1px 2px rgba(15, 23, 42, 0.05);
}

.solution-switch {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  margin-left: auto;
}

.solution-switch input[type="checkbox"] {
  appearance: none;
  width: 38px;
  height: 22px;
  background: #e6eef8;
  border-radius: 999px;
  position: relative;
  cursor: pointer;
  box-shadow: inset 0 1px 2px rgba(2, 6, 23, 0.04);
  transition: background .15s;
}

.solution-switch input[type="checkbox"]::after {
  content: '';
  position: absolute;
  left: 3px;
  top: 3px;
  width: 16px;
  height: 16px;
  background: #fff;
  border-radius: 50%;
  box-shadow: 0 2px 6px rgba(2, 6, 23, 0.12);
  transition: transform .15s;
}

.solution-switch input[type="checkbox"]:focus-visible {
  outline: 2px solid #2563eb;
  outline-offset: 2px;
}

.solution-switch input[type="checkbox"]:checked {
  background: linear-gradient(90deg, #3b82f6, #06b6d4);
}

.solution-switch input[type="checkbox"]:checked::after {
  transform: translateX(16px);
}

.colab-output summary {
  cursor: pointer;
  font-weight: 700;
  color: #334155;
  padding: 0.45rem 0.6rem;
  border-radius: 6px;
  background: linear-gradient(90deg, #f8fafc, #f1f5f9);
  margin-top: 0.4rem;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.colab-output summary::before {
  content: "▸ Output";
  margin-right: 0.5rem;
}

.colab-output[open] summary::before {
  content: "▾ Output";
}

.output-content {
  background: #ffffff;
  border: 1px solid #e6eef8;
  padding: 0.8rem;
  border-radius: 6px;
  margin-top: 0.5rem;
  overflow: auto;
  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, 'Roboto Mono', 'Courier New', monospace;
  font-size: 0.9rem;
  color: #0f1724;
}

.output-switch {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  margin-left: auto;
}

.output-switch input[type="checkbox"] {
  appearance: none;
  width: 38px;
  height: 22px;
  background: #e6eef8;
  border-radius: 999px;
  position: relative;
  cursor: pointer;
  box-shadow: inset 0 1px 2px rgba(2, 6, 23, 0.04);
  transition: background .15s;
}

.output-switch input[type="checkbox"]::after {
  content: '';
  position: absolute;
  left: 3px;
  top: 3px;
  width: 16px;
  height: 16px;
  background: #fff;
  border-radius: 50%;
  box-shadow: 0 2px 6px rgba(2, 6, 23, 0.12);
  transition: transform .15s;
}

.output-switch input[type="checkbox"]:focus-visible {
  outline: 2px solid #2563eb;
  outline-offset: 2px;
}

.output-switch input[type="checkbox"]:checked {
  background: linear-gradient(90deg, #3b82f6, #06b6d4);
}

.output-switch input[type="checkbox"]:checked::after {
  transform: translateX(16px);
}

.back-to-top-btn {
  position: fixed;
  bottom: 2rem;
  right: 2rem;
  z-index: 1010;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 48px;
  height: 48px;
  border: 1px solid #cbd5e1;
  background: #ffffff;
  color: #334155;
  border-radius: 50%;
  cursor: pointer;
  box-shadow: 0 4px 14px rgba(15, 23, 42, 0.1);
  transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
  opacity: 0;
  pointer-events: none;
  transform: translateY(20px);
  overflow: hidden;
}

.back-to-top-btn::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: linear-gradient(135deg, #2563eb, #1d4ed8);
  opacity: 0;
  z-index: 1;
  transition: opacity 0.3s ease;
}

.back-to-top-btn.visible {
  opacity: 1;
  pointer-events: auto;
  transform: translateY(0);
}

.back-to-top-btn:hover {
  color: #ffffff;
  border-color: #2563eb;
  box-shadow: 0 6px 20px rgba(37, 99, 235, 0.35);
  transform: translateY(-4px);
}

.back-to-top-btn:hover::before {
  opacity: 1;
}

.back-to-top-btn>* {
  position: relative;
  z-index: 2;
}

.back-to-top-btn .arrow-icon {
  transition: transform 0.3s cubic-bezier(0.4, 0, 0.2, 1);
}

.back-to-top-btn:hover .arrow-icon {
  transform: translateY(-3px);
}

.back-to-top-btn:active {
  transform: translateY(-1px);
  box-shadow: 0 4px 10px rgba(37, 99, 235, 0.25);
}

.back-to-top-btn:focus-visible {
  outline: 2px solid #2563eb;
  outline-offset: 2px;
}

.practice-test-banner button {
  transition: all 0.2s ease;
}

.practice-test-banner button:hover {
  transform: translateY(-2px);
  box-shadow: 0 6px 20px rgba(16, 185, 129, 0.5) !important;
}

.practice-test-banner button:active {
  transform: translateY(0);
}

.section-controls a button {
  transition: all 0.2s ease;
}

.section-controls a button:hover {
  transform: translateY(-1px);
  box-shadow: 0 4px 12px rgba(5, 150, 105, 0.4) !important;
}

.footer-note {
  text-align: center;
  margin-top: 2rem;
}

/* Difficulty badge */
.difficulty-badge {
  position: absolute;
  top: 2px;
  right: 2px;
  font-size: 0.42rem;
  font-weight: 700;
  color: #fff;
  padding: 0 3px;
  border-radius: 2px;
  line-height: 1.3;
  letter-spacing: 0.02em;
  text-transform: uppercase;
  opacity: 0.9;
  pointer-events: none;
  white-space: nowrap;
  max-width: 7em;
  overflow: hidden;
  text-overflow: ellipsis;
  display: inline-block;
}

/* Sort controls */
.sort-controls {
  display: inline-flex;
  align-items: center;
  margin-left: auto;
  position: relative;
}

.sort-btn {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 30px;
  height: 30px;
  border: 1px solid #cbd5e1;
  border-radius: 6px;
  background: #fff;
  cursor: pointer;
  color: #334155;
  font-size: 1.1rem;
  line-height: 1;
  transition: all 0.15s;
  padding: 0;
}

.sort-btn:hover {
  background: #f1f5f9;
  border-color: #64748b;
  color: #0f172a;
}

.sort-btn:focus {
  outline: none;
  border-color: #2563eb;
  box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.12);
}

.sort-btn svg { display: block; }

.sort-dropdown {
  position: absolute;
  right: 0;
  top: calc(100% + 6px);
  background: #fff;
  border: 1px solid #e2e8f0;
  border-radius: 10px;
  box-shadow: 0 8px 24px rgba(2, 6, 23, 0.12);
  padding: 0.5rem;
  z-index: 1100;
  min-width: 160px;
  display: flex;
  flex-direction: column;
  gap: 0.25rem;
}

.sort-dropdown[hidden] { display: none; }

.sort-dropdown-header {
  font-size: 0.75rem;
  font-weight: 700;
  color: #94a3b8;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  padding: 0.3rem 0.5rem 0.4rem;
  border-bottom: 1px solid #f1f5f9;
  margin-bottom: 0.15rem;
}

.sort-option {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  padding: 0.35rem 0.5rem;
  border-radius: 6px;
  cursor: pointer;
  font-size: 0.85rem;
  color: #334155;
  transition: background 0.1s;
}

.sort-option:hover {
  background: #f1f5f9;
}

.sort-option input[type="radio"] {
  accent-color: #2563eb;
  margin: 0;
  flex-shrink: 0;
}

/* Difficulty legend */
.diff-legend {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  padding: 0.6rem 0.75rem;
  margin-top: 1rem;
  background: #f8fafc;
  border-radius: 8px;
  font-size: 0.78rem;
  flex-wrap: wrap;
}

.diff-legend-label {
  font-weight: 600;
  color: #475569;
  margin-right: 0.2rem;
}

.diff-legend-item {
  font-weight: 600;
  padding: 0.1rem 0.5rem;
  border-radius: 4px;
  color: #fff;
  font-size: 0.75rem;
  letter-spacing: 0.01em;
}

.diff-legend-item.diff-m { background: #059669; }
.diff-legend-item.diff-s { background: #2563eb; }
.diff-legend-item.diff-k { background: #d97706; }
.diff-legend-item.diff-c { background: #7c3aed; }

/* Correction badge & tooltip */
.correction-badge {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 22px;
  height: 22px;
  font-size: 0.9rem;
  font-weight: 700;
  color: #92400e;
  background: #fef3c7;
  border: 1px solid #fcd34d;
  cursor: pointer;
  border-radius: 50%;
  transition: all 0.15s;
  margin-left: 5px;
  vertical-align: middle;
  flex-shrink: 0;
  user-select: none;
  line-height: 1;
}

.correction-badge:hover {
  background: #fde68a;
  border-color: #f59e0b;
}

.correction-tooltip {
  display: none;
  z-index: 1000;
  background: #fff;
  border: 1px solid #f59e0b;
  border-radius: 8px;
  padding: 0.7rem 0.85rem;
  width: 280px;
  box-shadow: 0 6px 20px rgba(0,0,0,0.15);
  font-size: 0.82rem;
  line-height: 1.5;
  white-space: normal;
}

.correction-tooltip.open {
  display: block;
  position: absolute;
  top: 2rem;
  right: 0.75rem;
}

.corr-labeled {
  color: #dc2626;
  margin-bottom: 0.25rem;
}

.corr-correct {
  color: #059669;
  margin-bottom: 0.25rem;
}

.corr-reason {
  color: #64748b;
  font-style: italic;
  font-size: 0.78rem;
}