/* ================================================================
   Harvard GSD Style — 商业街区景观设计 课程平台
   Typography: Crimson Text (serif) + Inter (sans)
   Palette: Black / White / Gray + Harvard Crimson (#A51C30)
   ================================================================ */

/* ===== CSS Variables ===== */
:root {
  /* --- GSD Color System --- */
  --crimson: #A51C30;
  --crimson-dark: #8B0000;
  --crimson-light: rgba(165,28,48,.08);
  --ink: #1a1a1a;
  --ink-soft: #2d2d2d;
  --body: #3d3d3d;
  --muted: #6b6b6b;
  --light: #9a9a9a;
  --border: #d8d8d8;
  --border-light: #eee;
  --surface: #fafafa;
  --surface-alt: #f5f5f5;
  --white: #fff;

  /* --- Typography --- */
  --font-serif: 'Crimson Text', 'Noto Serif SC', Georgia, 'Times New Roman', serif;
  --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;

  /* --- Spacing & Shape --- */
  --radius-sm: 2px;
  --radius-md: 4px;
  --radius-lg: 6px;
  --shadow-xs: 0 1px 2px rgba(0,0,0,.06);
  --shadow-sm: 0 2px 8px rgba(0,0,0,.06);
  --shadow-md: 0 4px 16px rgba(0,0,0,.08);

  --ease: cubic-bezier(.25,.1,.25,1);
  --transition: .2s var(--ease);
  --navbar-height: 72px;
  --container-max: 1100px;
}

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

html { scroll-behavior: smooth; scroll-padding-top: var(--navbar-height) }

body {
  font-family: var(--font-sans);
  color: var(--body);
  background: var(--white);
  line-height: 1.7;
  font-size: 15px;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

img, svg { display: block; max-width: 100% }
a { text-decoration: none; color: inherit }
button { cursor: pointer; border: none; background: none; font-family: inherit; font-size: inherit }
.container { width: 100%; max-width: var(--container-max); margin: 0 auto; padding: 0 2rem }


/* ================================================================
   NAVBAR
   ================================================================ */
.navbar {
  position: fixed;
  top: 0; left: 0; right: 0;
  z-index: 1000;
  height: var(--navbar-height);
  background: rgba(255,255,255,.92);
  backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--border-light);
  transition: box-shadow var(--transition);
}
.navbar.scrolled { box-shadow: var(--shadow-xs) }

.navbar__container {
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: 100%;
  max-width: var(--container-max);
  margin: 0 auto;
  padding: 0 2rem;
}

.navbar__brand { display: flex; align-items: center; gap: 10px }
.navbar__logo-icon { width: 34px; height: 34px }

.navbar__course {
  display: block;
  font-family: var(--font-serif);
  font-size: 15px;
  font-weight: 700;
  color: var(--ink);
  letter-spacing: .01em;
}
.navbar__sub {
  display: block;
  font-size: 11px;
  color: var(--crimson);
  font-weight: 600;
  letter-spacing: .06em;
  text-transform: uppercase;
}

.navbar__toggle {
  display: none;
  flex-direction: column;
  gap: 5px;
  padding: 8px;
  width: 32px;
  height: 32px;
  justify-content: center;
  align-items: center;
}
.navbar__toggle span {
  display: block;
  width: 18px; height: 1.5px;
  background: var(--ink);
  transition: all .25s ease;
  transform-origin: center;
}
.navbar__toggle.active span:nth-child(1) { transform: rotate(45deg) translate(4px,4px) }
.navbar__toggle.active span:nth-child(2) { opacity: 0 }
.navbar__toggle.active span:nth-child(3) { transform: rotate(-45deg) translate(4px,-4px) }

.navbar__menu { display: flex; align-items: center; gap: 0 }

.navbar__link {
  padding: 8px 16px;
  font-size: 13px;
  font-weight: 500;
  color: var(--muted);
  letter-spacing: .03em;
  transition: color var(--transition);
  position: relative;
}
.navbar__link:hover,
.navbar__link.active { color: var(--ink) }
.navbar__link.active::after {
  content: '';
  position: absolute;
  bottom: -1px;
  left: 16px;
  right: 16px;
  height: 1.5px;
  background: var(--crimson);
}


/* ================================================================
   HERO — GSD Minimalist
   ================================================================ */
.hero {
  position: relative;
  min-height: 88vh;
  display: flex;
  align-items: center;
  padding: calc(var(--navbar-height) + 5rem) 0 5rem;
  background: var(--white);
}
.hero__bg { position: absolute; inset: 0; pointer-events: none }

.hero__content {
  position: relative;
  z-index: 2;
  max-width:720px;
}

.hero__label {
  font-family: var(--font-sans);
  font-size: 11px;
  font-weight: 600;
  letter-spacing: .15em;
  text-transform: uppercase;
  color: var(--crimson);
  margin-bottom: 1.75rem;
}

.hero__title {
  font-family: var(--font-serif);
  font-size: clamp(2.6rem, 5.5vw, 4.2rem);
  font-weight: 700;
  line-height: 1.12;
  color: var(--ink);
  letter-spacing: -.01em;
  margin-bottom: 1.5rem;
}

.text-accent {
  color: var(--crimson);
  -webkit-text-fill-color: var(--crimson);
}

.hero__divider {
  width: 48px;
  height: 1.5px;
  background: var(--crimson);
  margin-bottom: 1.75rem;
}

.hero__desc {
  font-size: 1.05rem;
  color: var(--muted);
  line-height: 1.85;
  max-width: 540px;
  margin-bottom: 2.5rem;
}

.hero__info-grid {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 1rem;
  max-width: 320px;
  margin-bottom: 2.5rem;
}

.hero__info-item {
  text-align: center;
  padding: 1.25rem 1rem;
  border: 1px solid var(--border);
  background: var(--white);
}

.hero__info-num {
  display: block;
  font-family: var(--font-serif);
  font-size: 1.65rem;
  font-weight: 700;
  color: var(--ink);
}

.hero__info-label {
  font-size: 11px;
  color: var(--light);
  font-weight: 500;
  letter-spacing: .04em;
  text-transform: uppercase;
  margin-top: 2px;
}

.hero__actions { display: flex; gap: 1rem; flex-wrap: wrap }


/* ================================================================
   BUTTONS — GSD Clean
   ================================================================ */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  padding: 13px 28px;
  font-size: 14px;
  font-weight: 600;
  letter-spacing: .03em;
  border-radius: var(--radius-sm);
  transition: all var(--transition);
  font-family: var(--font-sans);
}

.btn--primary {
  background: var(--ink);
  color: var(--white);
}
.btn--primary:hover {
  background: var(--crimson);
}

.btn--outline {
  border: 1.5px solid var(--border);
  color: var(--ink);
  background: transparent;
}
.btn--outline:hover {
  border-color: var(--ink);
  background: var(--surface);
}

.btn--ghost {
  color: var(--muted);
  background: transparent;
}
.btn--ghost:hover { color: var(--ink) }

.btn--lg { padding: 15px 34px; font-size: 14px }
.btn:hover svg { transform: translateX(3px) }
.btn svg { transition: transform .15s ease }


/* ================================================================
   SECTION — Academic Spacing
   ================================================================ */
.section { padding: 5rem 0 }
.section--gray { background: var(--surface) }
.section--dark {
  background: var(--ink);
  color: var(--white);
}
.section--dark .section-title,
.section--dark .section-desc { color: var(--white) }
.section--dark .section-desc { color: rgba(255,255,255,.55) }

.section-header {
  text-align: center;
  max-width: 600px;
  margin: 0 auto 3.5rem;
}
.section-header--left { text-align: left; margin-left: 0 }

.section-badge {
  display: inline-flex;
  width: 44px;
  height: 44px;
  font-family: var(--font-serif);
  font-size: 18px;
  font-weight: 700;
  color: var(--crimson);
  border: 1.5px solid var(--border);
  margin-bottom: 1.5rem;
  align-items: center;
  justify-content: center;
  border-radius: var(--radius-sm);
}
.section-badge--green,
.section-badge--blue,
.section-badge--amber,
.section-badge--purple,
.section-badge--accent {
  background: transparent;
  color: var(--crimson);
  border-color: var(--crimson);
}

.section-title {
  font-family: var(--font-serif);
  font-size: clamp(1.8rem, 3.5vw, 2.6rem);
  font-weight: 700;
  color: var(--ink);
  letter-spacing: -.01em;
  line-height: 1.2;
  margin-bottom: 1rem;
}

.section-desc {
  font-size: 1rem;
  color: var(--muted);
  line-height: 1.75;
}


/* ================================================================
   INTRO CARDS — Border-based, No Heavy Shadows
   ================================================================ */
.intro-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 1.25rem;
}

.intro-card {
  background: var(--white);
  border-radius: var(--radius-md);
  padding: 2rem;
  border: 1px solid var(--border-light);
  transition: border-color var(--transition), box-shadow var(--transition);
}
.intro-card:hover {
  border-color: var(--border);
  box-shadow: var(--shadow-sm);
}
.intro-card--main { grid-column: 1 / -1 }
.intro-card--highlight {
  background: var(--surface);
  border-color: var(--border-light);
}

.intro-card__icon {
  width: 48px;
  height: 48px;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border);
  color: var(--ink);
  margin-bottom: 1.5rem;
}
.intro-card__icon svg { width: 24px; height: 24px }
.intro-card__icon--blue,
.intro-card__icon--amber,
.intro-card__icon--purple {
  border-color: var(--border);
  color: var(--ink);
  background: transparent;
}

.intro-card__title {
  font-family: var(--font-serif);
  font-size: 1.2rem;
  font-weight: 700;
  color: var(--ink);
  margin-bottom: 1rem;
}

.intro-card__text {
  font-size: .93rem;
  color: var(--body);
  line-height: 1.8;
}

.intro-card__list li {
  position: relative;
  padding: 5px 0 5px 20px;
  font-size: .93rem;
  color: var(--body);
  line-height: 1.65;
}
.intro-card__list li::before {
  content: '';
  position: absolute;
  left: 0;
  top: 12px;
  width: 5px;
  height: 5px;
  background: var(--crimson);
}

.intro-card__tags { display: flex; flex-wrap: wrap; gap: 8px }

.tag {
  display: inline-block;
  padding: 5px 14px;
  font-size: 12px;
  font-weight: 500;
  letter-spacing: .03em;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border);
  color: var(--muted);
  background: var(--white);
}
.tag--green, .tag--blue, .tag--amber, .tag--purple, .tag--teal, .tag--outline {
  border-color: var(--border);
  color: var(--body);
  background: var(--white);
}

.feature-highlights { display: flex; flex-direction: column; gap: 1.25rem }

.feature-highlight { display: flex; align-items: flex-start; gap: 1rem }
.feature-highlight__num {
  font-family: var(--font-serif);
  font-size: 1.4rem;
  font-weight: 700;
  color: var(--crimson);
  opacity: .35;
  line-height: 1;
  min-width: 36px;
}
.feature-highlight__content strong {
  font-family: var(--font-serif);
  font-size: .95rem;
  color: var(--ink);
  display: block;
  margin-bottom: 2px;
}
.feature-highlight__content span {
  font-size: .86rem;
  color: var(--muted);
  line-height: 1.6;
}


/* ================================================================
   KNOWLEDGE GRAPH — Canvas Force-Directed Interactive
   Draggable nodes, zoom, hover info, toolbar controls
   ================================================================ */

/* Container */
.graph-container {
  position: relative;
  background: rgba(255,255,255,.03);
  border: 1px solid rgba(255,255,255,.08);
  border-radius: var(--radius-md);
  overflow: hidden;
}

/* Toolbar */
.graph-toolbar {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 10px 16px;
  border-bottom: 1px solid rgba(255,255,255,.08);
  background: rgba(0,0,0,.2);
}
.btn-sm {
  padding: 5px 14px;
  font-size: 12px;
  font-weight: 600;
  border-radius: var(--radius-sm);
  border: 1px solid rgba(255,255,255,.15);
  color: rgba(255,255,255,.7);
  background: transparent;
  cursor: pointer;
  transition: all .2s var(--ease);
  font-family: var(--font-sans);
  letter-spacing: .02em;
}
.btn-sm:hover {
  border-color: var(--crimson);
  color: #fff;
  background: rgba(165,28,48,.12);
}
.graph-hint {
  font-size: 11px;
  color: rgba(255,255,255,.35);
  letter-spacing: .03em;
}

/* Canvas Area */
.graph-canvas {
  width: 100%;
  height: 520px;
  cursor: grab;
  position: relative;
  overflow: hidden;
  background:
    radial-gradient(ellipse 45% 55% at 50% 48%, rgba(165,28,48,.05) 0%, transparent 65%);
}
.graph-canvas:active { cursor: grabbing }
.graph-canvas canvas { width: 100%; height: 100%; display: block }

/* Info Panel (on hover) */
.graph-info {
  position: absolute;
  bottom: 16px; left: 16px;
  background: rgba(26,26,26,.92);
  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);
  border: 1px solid rgba(255,255,255,.1);
  border-radius: var(--radius-md);
  padding: 12px 18px;
  min-width: 220px;
  max-width: 300px;
  box-shadow: 0 8px 24px rgba(0,0,0,.3);
  pointer-events: none;
  opacity: 0;
  transform: translateY(8px);
  transition: all .25s var(--ease);
}
.graph-info.visible {
  opacity: 1;
  transform: translateY(0);
  pointer-events: auto;
}
.graph-info h4 {
  font-family: var(--font-serif);
  font-size: 14px;
  font-weight: 700;
  color: #fff;
  margin-bottom: 4px;
}
.graph-info p {
  font-size: 12.5px;
  color: rgba(255,255,255,.6);
  line-height: 1.65;
}

/* Legend */
.graph-legend {
  position: absolute;
  top: 56px; right: 14px;
  background: rgba(26,26,26,.85);
  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);
  border: 1px solid rgba(255,255,255,.08);
  border-radius: var(--radius-md);
  padding: 10px 14px;
  font-size: 11.5px;
  box-shadow: 0 4px 16px rgba(0,0,0,.2);
}
.graph-legend-item {
  display: flex;
  align-items: center;
  gap: 7px;
  padding: 3px 0;
  color: rgba(255,255,255,.55);
  letter-spacing: .02em;
}
.graph-legend-item .dot {
  width: 9px; height: 9px;
  border-radius: 50%;
  flex-shrink: 0;
}

@media(prefers-reduced-motion:reduce){ .graph-info{transition:none!important} }


/* ================================================================
   TIMELINE — Clean Vertical
   ================================================================ */
.timeline {
  position: relative;
  max-width: 800px;
  margin: 0 auto;
  padding-left: 48px;
}
.timeline::before {
  content: '';
  position: absolute;
  left: 17px;
  top: 0;
  bottom: 0;
  width: 1px;
  background: var(--border);
}
.timeline__item { position: relative; margin-bottom: 2rem }
.timeline__item:last-child { margin-bottom: 0 }

.timeline__marker {
  position: absolute;
  left: -48px; top: 4px;
  width: 34px; height: 34px;
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  background: var(--white);
  border: 1.5px solid var(--border);
  z-index: 2;
}
.timeline__marker--active {
  background: var(--crimson);
  border-color: var(--crimson);
}
.timeline__num {
  font-size: 12px;
  font-weight: 700;
  color: var(--muted);
}
.timeline__marker--active .timeline__num { color: var(--white) }

.timeline__card {
  background: var(--white);
  border-radius: var(--radius-md);
  padding: 1.75rem;
  border: 1px solid var(--border-light);
  transition: box-shadow var(--transition), border-color var(--transition);
}
.timeline__card:hover {
  border-color: var(--border);
  box-shadow: var(--shadow-sm);
}

.timeline__phase {
  font-size: 11px;
  font-weight: 600;
  color: var(--crimson);
  letter-spacing: .08em;
  text-transform: uppercase;
  margin-bottom: .5rem;
}

.timeline__title {
  font-family: var(--font-serif);
  font-size: 1.18rem;
  font-weight: 700;
  color: var(--ink);
  margin-bottom: 1rem;
}

.timeline__tasks { display: flex; flex-direction: column; gap: 5px; margin-bottom: 1.25rem }
.timeline__tasks li {
  position: relative;
  padding-left: 20px;
  font-size: .92rem;
  color: var(--body);
  line-height: 1.62;
}
.timeline__tasks li::before {
  content: '';
  position: absolute;
  left: 3px;
  top: 9px;
  width: 5px;
  height: 5px;
  border-radius: 1px;
  background: var(--crimson);
  opacity: .5;
}

.timeline__deliverable {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 8px 16px;
  background: var(--surface);
  border: 1px solid var(--border-light);
  border-radius: var(--radius-sm);
  font-size: .85rem;
  font-weight: 600;
  color: var(--ink);
}


/* ================================================================
   HOMEWORK & EVALUATION
   ================================================================ */
.homework-grid {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 1.25rem;
  margin-bottom: 3rem;
}

.hw-card {
  background: var(--white);
  border-radius: var(--radius-md);
  overflow: hidden;
  border: 1px solid var(--border-light);
  transition: border-color var(--transition), box-shadow var(--transition);
}
.hw-card:hover {
  border-color: var(--border);
  box-shadow: var(--shadow-sm);
}

.hw-card__header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: .7rem 1.5rem;
  color: var(--white);
  font-size: 13px;
}
.hw-card__header--green { background: var(--ink) }
.hw-card__header--blue { background: #333 }
.hw-card__header--amber { background: #444 }
.hw-card__header--purple { background: #2d2d2d }

.hw-card__num { font-size: 13px; font-weight: 700; letter-spacing: .03em }
.hw-card__type {
  font-size: 11px;
  font-weight: 500;
  opacity: .75;
  background: rgba(255,255,255,.15);
  padding: 2px 10px;
  border-radius: var(--radius-sm);
}

.hw-card__body { padding: 1.5rem }
.hw-card__title {
  font-family: var(--font-serif);
  font-size: 1.1rem;
  font-weight: 700;
  color: var(--ink);
  margin-bottom: .5rem;
}
.hw-card__desc {
  font-size: .9rem;
  color: var(--body);
  line-height: 1.7;
  margin-bottom: 1rem;
}

.hw-card__meta { display: flex; gap: 1.5rem; margin-bottom: 1rem }
.hw-card__meta span {
  display: flex;
  align-items: center;
  gap: 5px;
  font-size: .82rem;
  color: var(--muted);
}

.hw-card__requirements li {
  position: relative;
  padding: 3px 0 3px 18px;
  font-size: .86rem;
  color: var(--body);
  line-height: 1.6;
}
.hw-card__requirements li::before {
  content: '\2713';
  position: absolute;
  left: 0;
  color: var(--crimson);
  font-weight: 700;
  font-size: .8rem;
}

.evaluation-section {
  background: var(--white);
  border-radius: var(--radius-md);
  padding: 2rem;
  border: 1px solid var(--border-light);
}

.evaluation-title {
  display: flex;
  align-items: center;
  gap: 10px;
  font-family: var(--font-serif);
  font-size: 1.15rem;
  font-weight: 700;
  margin-bottom: 1.5rem;
}
.evaluation-title svg { color: var(--crimson) }

.evaluation-bars { display: flex; flex-direction: column; gap: 1.2rem; margin-bottom: 1rem }

.eval-bar-info {
  display: flex;
  justify-content: space-between;
  margin-bottom: 5px;
}
.eval-bar-info span { font-size: .88rem; color: var(--body) }
.eval-bar-info strong { font-size: .88rem; font-weight: 700; color: var(--ink) }

.eval-bar {
  height: 6px;
  background: var(--surface);
  border-radius: 3px;
  overflow: hidden;
}
.eval-bar__fill {
  height: 100%;
  border-radius: 3px;
  background: var(--ink);
  transition: width 1s var(--ease);
}
.eval-bar--purple,
.eval-bar--amber,
.eval-bar--blue,
.eval-bar--green { background: var(--ink) }

.evaluation-note {
  padding-top: 1.25rem;
  border-top: 1px solid var(--border-light);
}
.evaluation-note p { font-size: .86rem; color: var(--muted) }


/* ================================================================
   AI ASSISTANT — Understated
   ================================================================ */
.ai-section { background: var(--surface) }

.ai-layout {
  display: grid;
  grid-template-columns: 1fr 360px;
  gap: 1.5rem;
  align-items: start;
}

.chat-panel {
  background: var(--white);
  border-radius: var(--radius-md);
  border: 1px solid var(--border-light);
  overflow: hidden;
  box-shadow: var(--shadow-xs);
}

.chat-panel__header {
  display: flex;
  align-items: center;
  gap: 1rem;
  padding: 1.25rem;
  border-bottom: 1px solid var(--border-light);
}
.chat-name {
  font-family: var(--font-serif);
  font-size: 1rem;
  font-weight: 700;
  color: var(--ink);
}
.chat-status {
  display: flex;
  align-items: center;
  gap: 6px;
  font-size: .8rem;
  color: var(--muted);
}
.status-dot {
  width: 6px; height: 6px;
  border-radius: 50%;
  background: var(--crimson);
}

.chat-messages {
  height: 380px;
  overflow-y: auto;
  padding: 1.25rem;
  display: flex;
  flex-direction: column;
  gap: 1rem;
}
.chat-msg {
  display: flex;
  gap: 12px;
  max-width: 85%;
  animation: msgIn .3s var(--ease);
}
.chat-msg--user { align-self: flex-end; flex-direction: row-reverse }
.chat-msg__avatar { flex-shrink: 0; width: 34px; height: 34px; border-radius: 50%; overflow: hidden }
.chat-msg__bubble {
  padding: 12px 16px;
  border-radius: var(--radius-md);
  font-size: .9rem;
  line-height: 1.7;
}
.chat-msg--ai .chat-msg__bubble {
  background: var(--surface);
  color: var(--body);
  border-bottom-left-radius: 2px;
}
.chat-msg--user .chat-msg__bubble {
  background: var(--ink);
  color: var(--white);
  border-bottom-right-radius: 2px;
}
.chat-msg__list { margin: 8px 0; padding-left: 16px; list-style: disc }
.chat-msg__list li { font-size: .87rem; line-height: 1.58; margin-bottom: 3px }

.chat-input-area {
  border-top: 1px solid var(--border-light);
  padding: 1rem;
}
.chat-suggestions { display: flex; flex-wrap: wrap; gap: 6px; margin-bottom: .5rem }
.chat-tag {
  padding: 5px 12px;
  font-size: .77rem;
  font-weight: 500;
  color: var(--body);
  background: var(--white);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  white-space: nowrap;
  transition: border-color var(--transition);
}
.chat-tag:hover { border-color: var(--crimson); color: var(--crimson) }

.chat-form { display: flex; gap: 8px }
.chat-input {
  flex: 1;
  padding: 10px 16px;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  font-size: .9rem;
  outline: none;
  font-family: var(--font-sans);
  color: var(--ink);
  background: var(--white);
  transition: border-color var(--transition);
}
.chat-input:focus { border-color: var(--ink); box-shadow: none }
.chat-send {
  width: 40px; height: 40px;
  display: flex;
  align-items: center;
  justify-content: center;
  background: var(--ink);
  color: var(--white);
  border-radius: var(--radius-sm);
  flex-shrink: 0;
  transition: background var(--transition);
}
.chat-send:hover { background: var(--crimson) }

.ai-features { display: grid; grid-template-columns: 1fr 1fr; gap: .75rem }

.ai-feature-card {
  background: var(--white);
  border-radius: var(--radius-md);
  padding: 1.5rem;
  border: 1px solid var(--border-light);
  transition: border-color var(--transition), box-shadow var(--transition);
}
.ai-feature-card:hover {
  border-color: var(--border);
  box-shadow: var(--shadow-sm);
}

.ai-feature-icon {
  width: 42px; height: 42px;
  display: flex; align-items: center; justify-content: center;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border);
  margin-bottom: 1rem;
  color: var(--ink);
}
.ai-feature-icon svg { width: 20px; height: 20px }
.ai-feature-icon--cyan,
.ai-feature-icon--indigo,
.ai-feature-icon--emerald,
.ai-feature-icon--rose {
  border-color: var(--border);
  color: var(--ink);
  background: transparent;
}

.ai-feature-card h4 {
  font-family: var(--font-serif);
  font-size: .96rem;
  font-weight: 700;
  color: var(--ink);
  margin-bottom: 5px;
}
.ai-feature-card p { font-size: .84rem; color: var(--muted); line-height: 1.6 }


/* ================================================================
   FOOTER — Minimal Dark
   ================================================================ */
.footer {
  background: var(--ink);
  color: rgba(255,255,255,.4);
  padding: 3rem 0 1.25rem;
}
.footer__inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1.5rem;
  flex-wrap: wrap;
}
.footer__brand { display: flex; align-items: center; gap: 14px }
.footer__logo { width: 34px; height: 34px }
.footer__course {
  font-family: var(--font-serif);
  font-size: 1rem;
  font-weight: 700;
  color: var(--white);
}
.footer__dept { font-size: .82rem; color: rgba(255,255,255,.35) }

.footer__links { display: flex; gap: 1.5rem }
.footer__links a {
  font-size: .86rem;
  color: rgba(255,255,255,.4);
  transition: color var(--transition);
}
.footer__links a:hover { color: var(--white) }

.footer__copy {
  width: 100%;
  text-align: center;
  font-size: .8rem;
  color: rgba(255,255,255,.25);
  padding-top: 1.25rem;
  border-top: 1px solid rgba(255,255,255,.07);
  margin-top: 1.25rem;
}


/* ================================================================
   ANIMATIONS (minimal)
   ================================================================ */
@keyframes fadeInUp {
  from { opacity: 0; transform: translateY(20px) }
  to { opacity: 1; transform: translateY(0) }
}
@keyframes msgIn {
  from { opacity: 0; transform: translateY(8px) }
  to { opacity: 1; transform: translateY(0) }
}
@keyframes pulse-dot {
  0%,100% { opacity: 1 }
  50% { opacity: .4 }
}

[data-aos] {
  opacity: 0;
  transform: translateY(20px);
  transition: opacity .6s var(--ease), transform .6s var(--ease);
}
[data-aos].aos-animate { opacity: 1; transform: none }


/* ================================================================
   RESPONSIVE
   ================================================================ */
@media (max-width:1024px) {
  .ai-layout { grid-template-columns: 1fr }
  .ai-features { order: -1; grid-template-columns: repeat(4,1fr) }
  .hero__info-grid { grid-template-columns: repeat(2,1fr) }
}

@media (max-width:768px) {
  :root { --navbar-height: 64px }
  .container { padding: 0 1.25rem }

  .navbar__menu {
    display: none;
    position: fixed;
    top: 64px; left: 0; right: 0;
    background: var(--white);
    flex-direction: column;
    padding: 1.25rem;
    box-shadow: var(--shadow-md);
    z-index: 999;
  }
  .navbar__menu.active { display: flex }
  .navbar__link { width: 100%; padding: 12px 16px; text-align: center }
  .navbar__link.active::after { display: none }
  .navbar__toggle { display: flex }

  .hero { min-height: auto; padding: calc(64px+3rem) 0 3rem }
  .hero__title { font-size: clamp(2rem,7vw,3rem) }

  .intro-grid, .homework-grid { grid-template-columns: 1fr }
  .intro-card--main { grid-column: auto }

  .timeline { padding-left: 36px }
  .timeline__marker { left:-36px; width:28px; height:28px }
  .ai-features { grid-template-columns: 1fr 1fr }

  .footer__inner { flex-direction: column; text-align: center }
  .footer__links { flex-wrap: wrap; justify-content: center }
}

@media (max-width:480px) {
  .hero__actions { flex-direction: column }
  .btn--lg { width: 100% }
  .section { padding: 3.5rem 0 }
  .section-title { font-size: 1.6rem }
  .chat-messages { height: 280px }
  .ai-features { grid-template-columns: 1fr }
  .graph-canvas { height: 420px }
  .hero__info-grid { grid-template-columns: 1fr 1fr }
}

@media print {
  .navbar, .chat-input-area, .chat-send, .navbar__toggle { display: none!important }
  body { color: #000; background: #fff }
  .section { break-inside: avoid }
}

@media(prefers-reduced-motion:reduce) {
  *, *::before, *::after {
    animation-duration: .01ms!important;
    transition-duration: .01ms!important;
    scroll-behavior: auto!important;
  }
}