:root {
  color-scheme: light;
  --bg: #f6f7f8;
  --paper: #ffffff;
  --paper-soft: #f1f5f4;
  --quiet-bg: #ffffff;
  --ink: #172026;
  --muted: #65727c;
  --line: #d9e1e4;
  --teal: #0f766e;
  --teal-ink: #074a46;
  --amber: #c46a2b;
  --rose: #b83a48;
  --blue: #34526f;
  --header-bg: rgba(255, 255, 255, 0.88);
  --header-line: rgba(217, 225, 228, 0.86);
  --accent-soft: #eef7f5;
  --chip-line: rgba(15, 118, 110, 0.18);
  --tooltip-bg: #fff0e7;
  --tooltip-ink: #86400d;
  --result-bg: #eef7f5;
  --grid-line: rgba(15, 118, 110, 0.08);
  --circuit-line: rgba(52, 82, 111, 0.08);
  --image-bg: #ffffff;
  --card-shadow: 0 10px 28px rgba(20, 39, 52, 0.07);
  --shadow: 0 18px 45px rgba(20, 39, 52, 0.12);
  --radius: 8px;
}

:root[data-theme="dark"] {
  color-scheme: dark;
  --bg: #0d1214;
  --paper: #121a1d;
  --paper-soft: #172327;
  --quiet-bg: #101719;
  --ink: #eef6f4;
  --muted: #9fb0b4;
  --line: #27373b;
  --teal: #34d3c4;
  --teal-ink: #86fff0;
  --amber: #f0a86b;
  --rose: #ff8393;
  --blue: #8fb4d9;
  --header-bg: rgba(13, 18, 20, 0.82);
  --header-line: rgba(95, 128, 132, 0.26);
  --accent-soft: rgba(52, 211, 196, 0.12);
  --chip-line: rgba(52, 211, 196, 0.28);
  --tooltip-bg: #291a13;
  --tooltip-ink: #ffd2b0;
  --result-bg: rgba(52, 211, 196, 0.14);
  --grid-line: rgba(52, 211, 196, 0.08);
  --circuit-line: rgba(240, 168, 107, 0.08);
  --image-bg: #0f1517;
  --card-shadow: 0 14px 32px rgba(0, 0, 0, 0.28);
  --shadow: 0 24px 56px rgba(0, 0, 0, 0.42);
}

* {
  box-sizing: border-box;
}

html {
  font-size: calc(16px * var(--font-scale, 1));
  scroll-behavior: smooth;
}

body {
  position: relative;
  margin: 0;
  min-height: 100vh;
  overflow-x: hidden;
  background:
    linear-gradient(90deg, var(--grid-line) 1px, transparent 1px),
    linear-gradient(0deg, var(--grid-line) 1px, transparent 1px),
    var(--bg);
  background-size: 52px 52px, 52px 52px, auto;
  color: var(--ink);
  font-family:
    "Segoe UI", Arial, "Microsoft YaHei", "PingFang SC", "Helvetica Neue",
    sans-serif;
  line-height: 1.6;
  transition:
    background-color 260ms ease,
    color 260ms ease;
}

body::before {
  position: fixed;
  inset: 0;
  z-index: 0;
  background:
    linear-gradient(90deg, transparent 0 14%, var(--circuit-line) 14% 14.3%, transparent 14.3% 100%),
    linear-gradient(180deg, transparent 0 28%, var(--circuit-line) 28% 28.25%, transparent 28.25% 100%),
    linear-gradient(90deg, transparent 0 68%, var(--circuit-line) 68% 68.2%, transparent 68.2% 100%);
  background-size: 420px 420px, 360px 360px, 520px 520px;
  content: "";
  opacity: 0.8;
  pointer-events: none;
  animation: circuitDrift 24s linear infinite;
}

body > * {
  position: relative;
  z-index: 1;
}

button,
input {
  font: inherit;
}

img {
  display: block;
  max-width: 100%;
}

.site-header {
  position: sticky;
  top: 0;
  z-index: 20;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 20px;
  padding: 14px 32px;
  border-bottom: 1px solid var(--header-line);
  background: var(--header-bg);
  backdrop-filter: blur(18px);
  box-shadow: 0 12px 32px rgba(20, 39, 52, 0.06);
}

.brand {
  display: inline-flex;
  align-items: center;
  gap: 12px;
  color: var(--ink);
  text-decoration: none;
}

.brand-mark {
  display: grid;
  width: 36px;
  height: 36px;
  place-items: center;
  border-radius: 50%;
  background: var(--teal);
  color: white;
  font-weight: 800;
  box-shadow: 0 0 0 4px var(--accent-soft);
}

.brand.home-brand {
  border-radius: 999px;
  transition:
    color 180ms ease,
    transform 180ms ease;
}

.brand.home-brand:hover,
.brand.home-brand:focus-visible {
  color: var(--teal-ink);
  transform: translateY(-1px);
}

.home-mark {
  overflow: hidden;
  transition:
    background 180ms ease,
    box-shadow 180ms ease,
    transform 180ms ease;
}

.home-mark svg {
  width: 20px;
  height: 20px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2;
  stroke-linecap: round;
  stroke-linejoin: round;
}

.brand.home-brand:hover .home-mark,
.brand.home-brand:focus-visible .home-mark {
  background: color-mix(in srgb, var(--teal) 88%, var(--amber));
  box-shadow:
    0 0 0 4px var(--accent-soft),
    0 12px 26px color-mix(in srgb, var(--teal) 34%, transparent);
  transform: rotate(-2deg);
}

.brand strong,
.brand small {
  display: block;
}

.brand small {
  color: var(--muted);
  font-size: 0.78rem;
}

.header-actions {
  display: flex;
  align-items: center;
  gap: 12px;
}

.top-nav {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
}

.top-nav a {
  min-width: 56px;
  padding: 7px 12px;
  border-radius: 999px;
  color: var(--muted);
  font-size: 14px;
  font-weight: 750;
  text-align: center;
  text-decoration: none;
  transition:
    background 180ms ease,
    color 180ms ease;
}

.top-nav a:hover,
.top-nav a:focus-visible {
  background: var(--paper-soft);
  color: var(--teal-ink);
}

.theme-toggle {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  min-height: 38px;
  padding: 5px 10px;
  border: 1px solid var(--line);
  border-radius: 999px;
  background: var(--paper);
  color: var(--muted);
  cursor: pointer;
  font-size: 14px;
  font-weight: 800;
  transition:
    border-color 180ms ease,
    color 180ms ease,
    transform 180ms ease,
    background 180ms ease;
}

.font-controls {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  padding: 3px;
  border: 1px solid var(--line);
  border-radius: 999px;
  background: var(--paper);
}

.font-button {
  min-width: 34px;
  min-height: 32px;
  border: 0;
  border-radius: 999px;
  background: transparent;
  color: var(--muted);
  cursor: pointer;
  font-size: 14px;
  font-weight: 900;
  transition:
    background 160ms ease,
    color 160ms ease,
    transform 160ms ease;
}

.font-button:hover,
.font-button:focus-visible {
  background: var(--accent-soft);
  color: var(--teal-ink);
  transform: translateY(-1px);
}

.theme-toggle:hover,
.theme-toggle:focus-visible {
  transform: translateY(-1px);
  border-color: var(--teal);
  color: var(--teal-ink);
}

.theme-toggle-track {
  position: relative;
  width: 42px;
  height: 22px;
  border-radius: 999px;
  background: var(--paper-soft);
  box-shadow: inset 0 0 0 1px var(--line);
}

.theme-toggle-thumb {
  position: absolute;
  top: 4px;
  left: 4px;
  width: 14px;
  height: 14px;
  border-radius: 50%;
  background: var(--teal);
  box-shadow: 0 0 12px rgba(15, 118, 110, 0.35);
  transition:
    left 220ms ease,
    background 220ms ease,
    box-shadow 220ms ease;
}

:root[data-theme="dark"] .theme-toggle-thumb {
  left: 24px;
  background: var(--amber);
  box-shadow: 0 0 16px rgba(240, 168, 107, 0.48);
}

.section-band {
  padding: 72px max(28px, calc((100vw - 1180px) / 2));
}

.section-band.quiet {
  background: transparent;
}

.section-band {
  position: relative;
}

#exercises {
  background: transparent;
}

#exercises::before {
  content: none;
}

#exercises > * {
  position: relative;
  z-index: 1;
}

.hero {
  position: relative;
  min-height: calc(100vh - 66px);
  display: grid;
  grid-template-columns: minmax(0, 1fr) minmax(360px, 0.85fr);
  align-items: center;
  gap: 44px;
  padding-top: 52px;
}

.hero::before {
  position: absolute;
  left: max(28px, calc((100vw - 1180px) / 2));
  right: max(28px, calc((100vw - 1180px) / 2));
  bottom: 42px;
  height: 1px;
  background: linear-gradient(90deg, transparent, var(--teal), var(--amber), transparent);
  content: "";
  opacity: 0.55;
  transform-origin: left;
  animation: pulseRail 3.8s ease-in-out infinite;
}

.hero h1 {
  max-width: 780px;
  margin: 0;
  font-size: 3.35rem;
  line-height: 1.06;
}

.hero-copy {
  animation: enter 520ms ease both;
}

.hero-slide {
  position: relative;
  margin: 0;
  overflow: hidden;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--paper);
  box-shadow: var(--shadow);
  animation: enter 650ms ease 80ms both;
}

.eyebrow {
  margin: 0 0 12px;
  color: var(--teal);
  font-size: 0.82rem;
  font-weight: 800;
  text-transform: uppercase;
}

.formula-ribbon {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  margin-top: 34px;
}

.formula-ribbon span,
.formula-pill {
  position: relative;
  display: inline-flex;
  align-items: center;
  min-height: 38px;
  padding: 7px 12px;
  border: 1px solid var(--chip-line);
  border-radius: 999px;
  background: var(--accent-soft);
  color: var(--teal-ink);
  font-family: "Segoe UI", Arial, "Microsoft YaHei", sans-serif;
  font-weight: 750;
}

.section-heading {
  display: flex;
  align-items: end;
  justify-content: space-between;
  gap: 24px;
  margin-bottom: 28px;
}

.section-heading h2 {
  margin: 0;
  font-size: 2rem;
  line-height: 1.18;
}

.translatable {
  position: relative;
  z-index: 2;
  display: inline;
  border-bottom: 1px dotted color-mix(in srgb, var(--teal) 65%, transparent);
  cursor: help;
  outline: none;
}

.action-button,
.slide-button,
.exercise-slide-button,
.mode-button,
.back-to-top {
  border: 1px solid var(--line);
  border-radius: 999px;
  background: var(--paper);
  color: var(--teal-ink);
  cursor: pointer;
  transition:
    transform 160ms ease,
    border-color 160ms ease,
    background 160ms ease,
    color 160ms ease;
}

.action-button.active,
.slide-button.active,
.mode-button.active {
  border-color: var(--chip-line);
  background: var(--teal);
  color: var(--bg);
}

.action-button:hover,
.slide-button:hover,
.mode-button:hover,
.back-to-top:hover {
  transform: translateY(-1px);
  border-color: var(--teal);
}

.translation {
  position: absolute;
  left: 0;
  bottom: calc(100% + 8px);
  z-index: 30;
  display: block;
  width: max-content;
  max-width: min(420px, 82vw);
  padding: 8px 11px;
  border: 1px solid rgba(196, 106, 43, 0.22);
  border-radius: var(--radius);
  background: var(--tooltip-bg);
  color: var(--tooltip-ink);
  box-shadow: var(--shadow);
  font-size: 0.88rem;
  font-weight: 650;
  line-height: 1.45;
  opacity: 0;
  pointer-events: none;
  transform: translateY(4px);
  transition:
    opacity 140ms ease,
    transform 140ms ease,
    visibility 140ms ease;
  visibility: hidden;
  white-space: normal;
}

.translation::after {
  position: absolute;
  left: 14px;
  bottom: -6px;
  width: 10px;
  height: 10px;
  border-right: 1px solid rgba(196, 106, 43, 0.22);
  border-bottom: 1px solid rgba(196, 106, 43, 0.22);
  background: var(--tooltip-bg);
  content: "";
  transform: rotate(45deg);
}

.translatable:hover .translation,
.translatable:focus .translation,
.translatable:focus-within .translation {
  opacity: 1;
  transform: translateY(0);
  visibility: visible;
}

.translatable:hover,
.translatable:focus,
.translatable:focus-within {
  z-index: 80;
}

.translatable:focus {
  border-bottom-color: var(--amber);
}

.roadmap {
  display: grid;
  grid-template-columns: repeat(5, minmax(0, 1fr));
  gap: 14px;
}

.roadmap a {
  position: relative;
  overflow: visible;
  min-height: 128px;
  padding: 16px;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--paper);
  color: var(--ink);
  text-decoration: none;
  box-shadow: var(--card-shadow);
  transition:
    transform 180ms ease,
    box-shadow 180ms ease,
    border-color 180ms ease;
}

.roadmap a::before,
.concept-card::before,
.exercise-card::before,
.lab-panel::before {
  position: absolute;
  inset: 0;
  border-radius: inherit;
  box-shadow: inset 0 1px 0 color-mix(in srgb, var(--teal) 36%, transparent);
  clip-path: inset(0 round var(--radius));
  content: "";
  opacity: 0;
  pointer-events: none;
  transition: opacity 180ms ease;
}

.roadmap a:hover::before,
.concept-card:hover::before,
.exercise-card:hover::before,
.lab-panel:hover::before {
  opacity: 1;
}

.roadmap a:hover,
.roadmap a:focus-visible {
  transform: translateY(-3px);
  border-color: var(--teal);
  box-shadow: var(--shadow);
}

.roadmap strong,
.roadmap span {
  display: block;
}

.roadmap strong {
  margin-bottom: 8px;
  color: var(--teal-ink);
  font-size: 0.95rem;
}

.roadmap span {
  color: var(--muted);
  font-size: 0.92rem;
}

.unit-list {
  display: grid;
  gap: 32px;
}

.unit {
  padding-top: 20px;
  border-top: 1px solid var(--line);
}

.unit-header {
  display: grid;
  grid-template-columns: minmax(0, 1fr) auto;
  gap: 20px;
  align-items: end;
  margin-bottom: 22px;
}

.unit-title {
  margin: 0;
  font-size: 1.72rem;
  line-height: 1.2;
}

.slide-range {
  color: var(--muted);
  font-size: 0.9rem;
}

.unit-grid {
  display: grid;
  grid-template-columns: minmax(0, 0.95fr) minmax(360px, 1.05fr);
  gap: 24px;
  align-items: start;
}

.concept-stack {
  display: grid;
  gap: 14px;
}

.concept-card,
.exercise-card,
.lab-panel {
  position: relative;
  overflow: visible;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--paper);
  box-shadow: var(--card-shadow);
  transition:
    border-color 180ms ease,
    box-shadow 180ms ease,
    transform 180ms ease,
    background 180ms ease;
}

.concept-card:hover,
.exercise-card:hover,
.lab-panel:hover {
  transform: translateY(-2px);
  border-color: color-mix(in srgb, var(--teal) 42%, var(--line));
  box-shadow: var(--shadow);
}

.revealable {
  opacity: 0;
  transform: translateY(18px) scale(0.985);
  transition:
    opacity 620ms cubic-bezier(0.2, 0.8, 0.2, 1),
    transform 620ms cubic-bezier(0.2, 0.8, 0.2, 1),
    filter 620ms ease;
  filter: blur(3px);
}

.revealable.is-visible {
  opacity: 1;
  transform: translateY(0) scale(1);
  filter: blur(0);
}

.concept-card {
  padding: 20px;
}

.concept-card h3 {
  margin: 0 0 12px;
  font-size: 1.05rem;
}

.concept-card ul {
  display: grid;
  gap: 10px;
  margin: 0;
  padding-left: 22px;
}

.concept-card li {
  color: var(--ink);
}

.formula-set {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin-top: 14px;
}

.term-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 8px;
  margin-top: 14px;
}

.term {
  padding: 10px;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--paper-soft);
}

.slide-viewer {
  position: sticky;
  top: 92px;
  overflow: hidden;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--paper);
  box-shadow: var(--shadow);
}

.slide-stage {
  min-height: 280px;
  background: var(--paper-soft);
}

.slide-stage img {
  width: 100%;
  aspect-ratio: 1403 / 992;
  object-fit: contain;
  background: var(--image-bg);
  cursor: zoom-in;
}

.slide-tools {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 10px;
  padding: 12px;
  border-top: 1px solid var(--line);
}

.slide-pages {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
}

.slide-button {
  width: 34px;
  height: 30px;
  font-size: 12px;
  font-weight: 750;
}

.slide-caption {
  color: var(--muted);
  font-size: 14px;
  white-space: nowrap;
}

.exercise-list {
  display: grid;
  gap: 22px;
}

.exercise-body {
  display: grid;
  grid-template-columns: minmax(0, 1fr) minmax(300px, 0.9fr);
  gap: 22px;
  padding: 20px;
}

.exercise-title {
  margin: 0 0 14px;
  font-size: 1.25rem;
}

.question-text {
  display: grid;
  gap: 10px;
  margin: 0;
  color: var(--ink);
}

.choice-list {
  display: grid;
  gap: 8px;
  margin: 14px 0 0;
  padding-left: 0;
  list-style: none;
}

.choice-list li {
  padding: 9px 10px;
  border-left: 3px solid var(--line);
  background: var(--paper-soft);
}

.question-slide {
  position: relative;
  align-self: start;
  overflow: hidden;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--image-bg);
}

.question-slide img {
  width: 100%;
  cursor: zoom-in;
}

.exercise-slide-controls {
  display: grid;
  gap: 10px;
  padding: 12px;
  border-top: 1px solid var(--line);
  background: var(--paper);
}

.exercise-slide-buttons {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
}

.exercise-slide-button {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  min-height: 32px;
  padding: 5px 10px;
  border: 1px solid var(--line);
  border-radius: 999px;
  background: var(--paper-soft);
  color: var(--teal-ink);
  cursor: pointer;
  font-size: 13px;
  font-weight: 850;
  transition:
    background 160ms ease,
    border-color 160ms ease,
    color 160ms ease,
    transform 160ms ease;
}

.exercise-slide-button span {
  color: var(--muted);
  font-size: 12px;
}

.exercise-slide-button.active {
  border-color: var(--teal);
  background: var(--teal);
  color: var(--bg);
}

.exercise-slide-button.active span {
  color: var(--bg);
}

.exercise-slide-button:hover,
.exercise-slide-button:focus-visible {
  transform: translateY(-1px);
  border-color: var(--teal);
}

.exercise-slide-slider {
  display: grid;
  gap: 6px;
  color: var(--muted);
  font-size: 13px;
  font-weight: 750;
}

.reveal-row {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  padding: 0 20px 18px;
}

.action-button {
  min-height: 36px;
  padding: 6px 13px;
  font-size: 14px;
  font-weight: 750;
}

.reveal-panel {
  display: none;
  margin: 0 20px 20px;
  padding: 18px;
  border: 1px solid var(--line);
  border-left: 4px solid var(--teal);
  border-radius: var(--radius);
  background: var(--paper-soft);
  animation: panelIn 220ms ease both;
}

.reveal-panel.active {
  display: block;
}

.reveal-panel h4 {
  margin: 0 0 8px;
  font-size: 16px;
}

.reveal-panel p,
.reveal-panel ol,
.solution-content {
  margin-bottom: 0;
}

.solution-content {
  display: grid;
  gap: 12px;
}

.solution-content p,
.solution-content ol {
  margin-top: 0;
}

.lab-grid {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 18px;
}

.deck-archive {
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--paper);
  box-shadow: var(--card-shadow);
}

.deck-archive summary {
  cursor: pointer;
  padding: 18px 20px;
  color: var(--teal-ink);
  font-weight: 850;
}

.deck-grid {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 12px;
  padding: 0 20px 20px;
}

.deck-thumb {
  overflow: hidden;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--image-bg);
}

.deck-thumb img {
  width: 100%;
  aspect-ratio: 1403 / 992;
  cursor: zoom-in;
  object-fit: cover;
}

.hero-slide img {
  cursor: zoom-in;
}

.image-modal {
  position: fixed;
  inset: 0;
  z-index: 200;
  display: grid;
  grid-template-rows: 1fr auto;
  gap: 14px;
  padding: 28px;
  background: rgba(4, 8, 10, 0.82);
  backdrop-filter: blur(18px);
  opacity: 0;
  pointer-events: none;
  transition: opacity 180ms ease;
}

.image-modal.active {
  opacity: 1;
  pointer-events: auto;
}

.image-modal img {
  align-self: center;
  justify-self: center;
  max-width: min(96vw, 1600px);
  max-height: calc(100vh - 112px);
  border: 1px solid rgba(255, 255, 255, 0.18);
  border-radius: var(--radius);
  background: #fff;
  box-shadow: 0 28px 90px rgba(0, 0, 0, 0.46);
  object-fit: contain;
}

.image-modal-caption {
  justify-self: center;
  max-width: min(96vw, 980px);
  color: rgba(255, 255, 255, 0.78);
  font-size: 14px;
  font-weight: 750;
  text-align: center;
}

.image-modal-close {
  position: absolute;
  top: 18px;
  right: 18px;
  width: 42px;
  height: 42px;
  border: 1px solid rgba(255, 255, 255, 0.24);
  border-radius: 50%;
  background: rgba(255, 255, 255, 0.08);
  color: white;
  cursor: pointer;
  font-size: 24px;
  line-height: 1;
}

.cursor-ripple {
  position: fixed;
  left: 0;
  top: 0;
  z-index: 190;
  width: 128px;
  height: 128px;
  border: 1px solid rgba(245, 176, 82, 0.38);
  border-radius: 50%;
  box-shadow:
    0 0 22px rgba(245, 176, 82, 0.18),
    inset 0 0 18px rgba(255, 214, 128, 0.13);
  pointer-events: none;
  animation: cursorRipple 620ms ease-out both;
}

.deck-thumb figcaption {
  padding: 7px 9px;
  color: var(--muted);
  font-size: 0.82rem;
  font-weight: 750;
}

.lab-panel {
  display: grid;
  gap: 14px;
  padding: 20px;
}

.lab-panel h3 {
  margin: 0;
  font-size: 1.15rem;
}

.lab-panel label {
  display: grid;
  gap: 6px;
  color: var(--ink);
  font-weight: 700;
}

input[type="range"] {
  accent-color: var(--teal);
}

input[type="number"] {
  width: 100%;
  padding: 10px 12px;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--paper-soft);
  color: var(--ink);
}

.segmented {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 8px;
}

.mode-button {
  min-height: 38px;
  font-size: 14px;
  font-weight: 800;
}

.result-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 8px;
}

.result-grid output,
.big-result {
  min-height: 54px;
  padding: 10px 12px;
  border-radius: var(--radius);
  background: var(--result-bg);
  color: var(--teal-ink);
  font-weight: 850;
}

.big-result {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  font-size: 1.25rem;
}

.back-to-top {
  position: fixed;
  right: 20px;
  bottom: 20px;
  display: none;
  width: 44px;
  height: 44px;
  border-radius: 50%;
  box-shadow: var(--shadow);
  font-size: 1.2rem;
  font-weight: 900;
}

.back-to-top.visible {
  display: block;
}

@keyframes enter {
  from {
    opacity: 0;
    transform: translateY(16px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

@keyframes circuitDrift {
  from {
    background-position: 0 0, 0 0, 0 0;
  }
  to {
    background-position: 420px 0, 0 360px, -520px 0;
  }
}

@keyframes cursorRipple {
  from {
    opacity: 0.26;
    transform: translate(-50%, -50%) scale(0.18);
  }
  to {
    opacity: 0;
    transform: translate(-50%, -50%) scale(1);
  }
}

@keyframes pulseRail {
  0%,
  100% {
    opacity: 0.22;
    transform: scaleX(0.35);
  }
  50% {
    opacity: 0.72;
    transform: scaleX(1);
  }
}

@keyframes panelIn {
  from {
    opacity: 0;
    transform: translateY(-4px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

@media (prefers-reduced-motion: reduce) {
  *,
  *::before,
  *::after {
    animation-duration: 1ms !important;
    animation-iteration-count: 1 !important;
    scroll-behavior: auto !important;
    transition-duration: 1ms !important;
  }
}

@media (max-width: 980px) {
  .site-header {
    align-items: flex-start;
    flex-direction: column;
    padding: 12px 20px;
  }

  .header-actions {
    align-items: flex-start;
    flex-direction: column;
  }

  .hero,
  .unit-grid,
  .exercise-body,
  .lab-grid {
    grid-template-columns: 1fr;
  }

  .hero {
    min-height: auto;
  }

  .hero h1 {
    font-size: 2.45rem;
  }

  .roadmap {
    grid-template-columns: repeat(2, minmax(0, 1fr));
  }

  .deck-grid {
    grid-template-columns: repeat(2, minmax(0, 1fr));
  }

  .slide-viewer {
    position: static;
  }
}

@media (max-width: 640px) {
  .section-band {
    padding: 48px 18px;
  }

  .hero h1 {
    font-size: 2rem;
  }

  .section-heading {
    align-items: start;
    flex-direction: column;
  }

  .roadmap,
  .deck-grid,
  .term-grid,
  .result-grid {
    grid-template-columns: 1fr;
  }

  .unit-header {
    grid-template-columns: 1fr;
  }

  .slide-tools {
    align-items: start;
    flex-direction: column;
  }

  .slide-caption {
    white-space: normal;
  }
}

.concept-help {
  margin-top: 14px;
  border: 1px solid color-mix(in srgb, var(--amber) 34%, var(--line));
  border-radius: calc(var(--radius) - 4px);
  background:
    linear-gradient(135deg, color-mix(in srgb, var(--amber) 9%, transparent), transparent 58%),
    color-mix(in srgb, var(--paper) 72%, transparent);
  overflow: hidden;
}

.concept-help summary {
  cursor: pointer;
  list-style: none;
  padding: 12px 14px;
  color: var(--teal-ink);
  font-size: calc(0.92rem * var(--font-scale));
  font-weight: 900;
}

.concept-help summary::-webkit-details-marker {
  display: none;
}

.concept-help summary::before {
  content: "+";
  display: inline-grid;
  width: 22px;
  height: 22px;
  margin-right: 8px;
  place-items: center;
  border: 1px solid var(--line);
  border-radius: 50%;
  color: var(--amber);
}

.concept-help[open] summary::before {
  content: "−";
}

.concept-help ol {
  display: grid;
  gap: 10px;
  margin: 0;
  padding: 0 18px 16px 38px;
  color: var(--ink);
  line-height: 1.76;
}

.concept-help-example {
  margin: 0 18px 18px;
  padding: 12px 14px;
  border-left: 3px solid var(--amber);
  border-radius: 10px;
  background: color-mix(in srgb, var(--amber) 8%, transparent);
  color: var(--muted);
  line-height: 1.72;
}

/* Tooltip layering fix for help blocks and dense cards. */
.concept-help,
.concept-help summary,
.concept-help .translatable,
.concept-card,
.unit,
.concept-stack {
  overflow: visible !important;
}

.concept-card:has(.translatable:hover),
.concept-card:has(.translatable:focus),
.concept-help:has(.translatable:hover),
.concept-help:has(.translatable:focus) {
  z-index: 1200;
}

.concept-help summary .translation {
  top: calc(100% + 10px);
  bottom: auto;
  z-index: 99999;
}

.concept-help summary .translation::after {
  top: -6px;
  bottom: auto;
}

.translation {
  z-index: 99999;
}


/* Tiny interactive help prompt. */
.mini-lab {
  margin-top: 12px;
  padding: 12px 14px;
  border: 1px solid rgba(255, 196, 99, 0.24);
  border-radius: 12px;
  background: linear-gradient(135deg, rgba(255, 196, 99, 0.08), rgba(118, 255, 239, 0.05));
  box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.08);
}

.mini-lab strong {
  display: inline-flex;
  margin-bottom: 6px;
  color: var(--accent-2);
  font-size: 0.88rem;
  letter-spacing: 0;
}

.mini-lab p {
  margin: 0;
  color: var(--muted);
  line-height: 1.75;
}


/* Exercise detail pacing for beginner-friendly explanations. */
.slow-detail {
  margin-top: 16px;
  padding: 16px 18px;
  border: 1px solid rgba(118, 255, 239, 0.18);
  border-radius: 14px;
  background: rgba(8, 26, 28, 0.5);
}

.slow-detail h4 {
  margin: 0 0 10px;
  color: var(--accent);
  font-size: 1.02rem;
  letter-spacing: 0;
}

.slow-detail p,
.slow-detail li {
  line-height: 1.9;
}

.slow-detail ol {
  margin: 10px 0;
  padding-left: 1.35rem;
}