:root {
  --ink: #131b2e;
  --muted: #3f4850;
  --outline: #707881;
  --outline-soft: #bfc7d2;
  --paper: #f8fafc;
  --card: #ffffff;
  --primary: #0284c7;
  --primary-deep: #006194;
  --primary-soft: #e0f2fe;
  --secondary: #059669;
  --secondary-soft: #d1fae5;
  --amber: #f59e0b;
  --shadow-1: 0 1px 3px rgba(15, 23, 42, 0.04), 0 6px 16px -2px rgba(15, 23, 42, 0.03);
  --shadow-2: 0 4px 12px -1px rgba(2, 132, 199, 0.12), 0 2px 6px -1px rgba(15, 23, 42, 0.04);
  --radius-card: 0.95rem;
  --radius-btn: 0.65rem;
  --font: "Plus Jakarta Sans", "Segoe UI", system-ui, sans-serif;
  --top-h: 4.1rem;
}

* { box-sizing: border-box; }

html { font-size: 15px; }

body {
  margin: 0;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  font-family: var(--font);
  color: var(--ink);
  background: var(--paper);
}

body::before {
  content: "";
  position: fixed;
  inset: 0;
  z-index: -1;
  background:
    radial-gradient(ellipse 70% 45% at 0% 0%, #dbeafe 0%, transparent 55%),
    radial-gradient(ellipse 50% 40% at 100% 0%, #d1fae5 0%, transparent 50%),
    linear-gradient(180deg, #f8fafc, #eef2ff 55%, #f8fafc);
}

.top {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 0.65rem;
  padding: 0.45rem 1rem;
  max-width: 1100px;
  width: 100%;
  margin: 0 auto;
  flex-wrap: wrap;
  min-height: var(--top-h);
}

.brand-block,
.top-left {
  display: flex;
  align-items: center;
  gap: 0.55rem;
}

.top-left {
  gap: 0;
}

.brand-logo {
  display: flex;
  align-items: center;
  flex-shrink: 0;
  line-height: 0;
  text-decoration: none;
}

.brand-logo img {
  display: block;
  height: 3.6rem;
  width: auto;
}

.brand-mark {
  width: 2.2rem;
  height: 2.2rem;
  border-radius: 0.7rem;
  display: grid;
  place-items: center;
  color: var(--primary-deep);
  background: var(--primary-soft);
}

.brand {
  font-weight: 800;
  letter-spacing: -0.02em;
  line-height: 1.1;
}

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

.subject-pill {
  position: relative;
  flex-shrink: 0;
  margin-left: 1.25rem;
  padding: 0.28rem 0.7rem;
  border-radius: 999px;
  font-size: 0.78rem;
  font-weight: 700;
  letter-spacing: -0.01em;
  color: var(--primary-deep);
  background: var(--primary-soft);
}

.subject-pill::before {
  content: "";
  position: absolute;
  left: -0.75rem;
  top: 18%;
  bottom: 18%;
  width: 1px;
  background: color-mix(in srgb, var(--outline-soft) 90%, transparent);
}

.top-left .tagline {
  margin-left: 0.55rem;
}

.top-nav {
  display: flex;
  gap: 0.35rem;
  flex-wrap: wrap;
}

.nav-item {
  text-decoration: none;
  color: var(--muted);
  font-weight: 600;
  font-size: 0.9rem;
  padding: 0.4rem 0.7rem;
  border-radius: 0.55rem;
}

.nav-item.active,
.nav-item[aria-current="page"] {
  color: var(--primary-deep);
  background: var(--primary-soft);
}

.library-main {
  flex: 1;
  width: min(1100px, 100%);
  margin: 0 auto;
  padding: 0.5rem 1rem 2rem;
}

.page-head {
  margin-bottom: 1.25rem;
}

.page-head h1 {
  margin: 0 0 0.35rem;
  font-size: clamp(1.6rem, 3vw, 2.1rem);
  letter-spacing: -0.03em;
}

.lede {
  margin: 0;
  color: var(--muted);
  line-height: 1.45;
}

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

.cards {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
  gap: 0.9rem;
}

.card {
  appearance: none;
  border: 1px solid var(--outline-soft);
  background: var(--card);
  border-radius: var(--radius-card);
  padding: 1rem 1.05rem 1.1rem;
  text-align: left;
  cursor: pointer;
  box-shadow: var(--shadow-1);
  display: grid;
  gap: 0.55rem;
  transition: transform 0.15s ease, box-shadow 0.15s ease, border-color 0.15s ease;
}

.card:hover,
.card:focus-visible {
  transform: translateY(-2px);
  box-shadow: var(--shadow-2);
  border-color: #93c5fd;
  outline: none;
}

.card-type {
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.04em;
  text-transform: uppercase;
  color: var(--primary-deep);
}

.card h2 {
  margin: 0;
  font-size: 1.05rem;
  line-height: 1.25;
  letter-spacing: -0.02em;
}

.card p {
  margin: 0;
  color: var(--muted);
  font-size: 0.9rem;
  line-height: 1.4;
}

.card-meta {
  display: flex;
  gap: 0.5rem;
  flex-wrap: wrap;
  font-size: 0.78rem;
  color: var(--outline);
  font-weight: 600;
}

.error {
  margin-top: 1rem;
  color: #ba1a1a;
}

.player { animation: fade-in 0.2s ease; }

@keyframes fade-in {
  from { opacity: 0; transform: translateY(6px); }
  to { opacity: 1; transform: none; }
}

.back-btn { margin-bottom: 0.75rem; }

.player-grid {
  display: grid;
  grid-template-columns: minmax(0, 1.6fr) minmax(240px, 0.9fr);
  gap: 1rem;
  align-items: start;
}

.player-stage,
.panel {
  background: var(--card);
  border: 1px solid var(--outline-soft);
  border-radius: var(--radius-card);
  box-shadow: var(--shadow-1);
}

.player-stage {
  padding: 1rem 1.1rem 1.15rem;
  display: grid;
  gap: 0.55rem;
}

.kicker {
  margin: 0;
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--primary);
}

.player-stage h2,
.panel h3 {
  margin: 0;
  letter-spacing: -0.02em;
}

.video-shell {
  background: #0f172a;
  border-radius: 0.8rem;
  overflow: hidden;
  aspect-ratio: 16 / 9;
}

video {
  width: 100%;
  height: 100%;
  display: block;
  object-fit: contain;
  background: #0f172a;
}

.player-side {
  display: grid;
  gap: 0.85rem;
}

.panel { padding: 1rem 1.05rem; display: grid; gap: 0.65rem; }

.meta {
  margin: 0;
  display: grid;
  gap: 0.45rem;
}

.meta div {
  display: grid;
  grid-template-columns: 4.5rem 1fr;
  gap: 0.4rem;
  font-size: 0.9rem;
}

.meta dt {
  margin: 0;
  color: var(--outline);
  font-weight: 600;
}

.meta dd { margin: 0; }

.policy {
  margin: 0;
  padding: 0.7rem 0.8rem;
  border-radius: 0.65rem;
  background: var(--secondary-soft);
  color: #065f46;
  font-size: 0.88rem;
  line-height: 1.4;
}

.equation {
  margin: 0;
  font-size: 1.35rem;
  font-weight: 600;
  letter-spacing: 0.02em;
}

.choices {
  display: grid;
  gap: 0.45rem;
}

.choice {
  display: flex;
  align-items: center;
  gap: 0.55rem;
  border: 1px solid var(--outline-soft);
  border-radius: 0.65rem;
  padding: 0.65rem 0.75rem;
  cursor: pointer;
  background: #fff;
}

.choice:has(input:checked) {
  border-color: var(--primary);
  background: var(--primary-soft);
}

.choice input { accent-color: var(--primary); }

.pause-feedback {
  min-height: 1.2em;
  margin: 0;
  font-size: 0.9rem;
  color: var(--muted);
}

.pause-feedback.ok { color: #065f46; }
.pause-feedback.bad { color: #9a3412; }

.actions {
  display: flex;
  flex-wrap: wrap;
  gap: 0.55rem;
}

button,
a.primary,
a.ghost {
  font: inherit;
  border-radius: var(--radius-btn);
  padding: 0.65rem 0.95rem;
  cursor: pointer;
  border: 0;
  text-decoration: none;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  font-weight: 700;
}

button:disabled { opacity: 0.45; cursor: default; }

.primary,
a.primary {
  background: var(--primary);
  color: #fff;
}

.ghost,
a.ghost,
.back-btn {
  background: transparent;
  color: var(--primary-deep);
  border: 1px solid var(--outline-soft);
}

@media (max-width: 820px) {
  .player-grid { grid-template-columns: 1fr; }
}

@media (max-width: 560px) {
  .top { padding: 0.55rem 0.75rem; }
  .library-main { padding: 0.25rem 0.75rem 1.5rem; }
  .cards { grid-template-columns: 1fr; }
}