:root {
  --color-bg: #f6f8fb;
  --color-surface: #ffffff;
  --color-surface-2: #f1f4f9;
  --color-text: #1b2433;
  --color-text-soft: #5b6473;
  --color-border: #e3e8ef;
  --color-primary: #2257d6;
  --color-primary-hover: #1a45b0;
  --color-primary-soft: #e6edff;
  --color-accent: #f59e0b;
  --color-success: #16a34a;
  --color-shadow: 0 6px 20px rgba(20, 34, 64, 0.08);
  --color-shadow-soft: 0 2px 6px rgba(20, 34, 64, 0.06);
  --radius-sm: 6px;
  --radius-md: 12px;
  --radius-lg: 18px;
  --font-sans: "Inter", "Segoe UI", "Roboto", "Helvetica Neue", Arial, sans-serif;
  --transition: 160ms ease;
  --container: 1180px;
}

* {
  box-sizing: border-box;
}

html, body {
  margin: 0;
  padding: 0;
  background: var(--color-bg);
  color: var(--color-text);
  font-family: var(--font-sans);
  font-size: 16px;
  line-height: 1.55;
  -webkit-font-smoothing: antialiased;
}

a {
  color: var(--color-primary);
  text-decoration: none;
}

a:hover {
  text-decoration: underline;
}

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

button {
  font-family: inherit;
  cursor: pointer;
}

.container {
  width: 100%;
  max-width: var(--container);
  margin: 0 auto;
  padding: 0 20px;
}

/* Header */
.site-header {
  position: sticky;
  top: 0;
  z-index: 50;
  background: var(--color-surface);
  border-bottom: 1px solid var(--color-border);
  box-shadow: var(--color-shadow-soft);
}

.site-header .container {
  display: flex;
  align-items: center;
  gap: 16px;
  padding-top: 12px;
  padding-bottom: 12px;
  flex-wrap: wrap;
}

.brand {
  display: flex;
  align-items: center;
  gap: 10px;
  font-weight: 700;
  font-size: 1.05rem;
  color: var(--color-text);
}

.brand-mark {
  width: 36px;
  height: 36px;
  border-radius: 10px;
  background: linear-gradient(135deg, #2257d6, #5b8def);
  display: inline-flex;
  align-items: center;
  justify-content: center;
  color: #fff;
  font-weight: 800;
}

.brand-text {
  display: flex;
  flex-direction: column;
  line-height: 1.1;
}

.brand-text small {
  font-weight: 500;
  font-size: 0.78rem;
  color: var(--color-text-soft);
}

.site-nav {
  margin-left: auto;
  display: flex;
  align-items: center;
  gap: 14px;
  flex-wrap: wrap;
}

.nav-link {
  font-size: 0.95rem;
  color: var(--color-text-soft);
  padding: 6px 10px;
  border-radius: var(--radius-sm);
  transition: background var(--transition), color var(--transition);
}

.nav-link:hover {
  background: var(--color-primary-soft);
  color: var(--color-primary);
  text-decoration: none;
}

.lang-switcher {
  display: inline-flex;
  background: var(--color-surface-2);
  border-radius: 999px;
  padding: 3px;
  gap: 2px;
}

.lang-btn {
  border: none;
  background: transparent;
  color: var(--color-text-soft);
  padding: 6px 12px;
  border-radius: 999px;
  font-size: 0.85rem;
  font-weight: 600;
  transition: background var(--transition), color var(--transition);
}

.lang-btn:hover {
  color: var(--color-primary);
}

.lang-btn.is-active {
  background: var(--color-primary);
  color: #fff;
  box-shadow: 0 1px 4px rgba(34, 87, 214, 0.35);
}

/* Hero */
.hero {
  padding: 56px 0 32px;
}

.hero-inner {
  display: grid;
  gap: 28px;
  grid-template-columns: 1.4fr 1fr;
  align-items: center;
}

.hero h1 {
  font-size: clamp(1.7rem, 3vw, 2.4rem);
  line-height: 1.15;
  margin: 0 0 14px;
  letter-spacing: -0.01em;
}

.hero p {
  margin: 0 0 20px;
  color: var(--color-text-soft);
  font-size: 1.05rem;
}

.hero-card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: 22px;
  box-shadow: var(--color-shadow);
}

.hero-card h3 {
  margin: 0 0 8px;
  font-size: 1.05rem;
}

.hero-card ul {
  margin: 0;
  padding-left: 20px;
  color: var(--color-text-soft);
}

.hero-card li + li {
  margin-top: 4px;
}

/* Search */
.search-section {
  padding: 12px 0 8px;
}

.search-box {
  position: relative;
  width: 100%;
  max-width: 520px;
}

.search-box input {
  width: 100%;
  padding: 12px 14px 12px 42px;
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
  background: var(--color-surface);
  font-size: 1rem;
  color: var(--color-text);
  box-shadow: var(--color-shadow-soft);
  transition: border var(--transition), box-shadow var(--transition);
}

.search-box input:focus {
  outline: none;
  border-color: var(--color-primary);
  box-shadow: 0 0 0 4px rgba(34, 87, 214, 0.15);
}

.search-box::before {
  content: "";
  position: absolute;
  left: 14px;
  top: 50%;
  width: 18px;
  height: 18px;
  transform: translateY(-50%);
  background: no-repeat center/contain url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="%235b6473" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><circle cx="11" cy="11" r="8"/><line x1="21" y1="21" x2="16.65" y2="16.65"/></svg>');
}

/* Lectures grid */
.lectures-section {
  padding: 24px 0 64px;
}

.section-head {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 12px;
  margin-bottom: 16px;
  flex-wrap: wrap;
}

.section-head h2 {
  margin: 0;
  font-size: 1.3rem;
}

.results-meta {
  color: var(--color-text-soft);
  font-size: 0.9rem;
}

.lecture-grid {
  display: grid;
  gap: 18px;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
}

.lecture-card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: 20px;
  display: flex;
  flex-direction: column;
  box-shadow: var(--color-shadow-soft);
  transition: transform var(--transition), box-shadow var(--transition), border-color var(--transition);
}

.lecture-card:hover {
  transform: translateY(-2px);
  box-shadow: var(--color-shadow);
  border-color: #c9d4e8;
}

.lecture-card h3 {
  margin: 0 0 10px;
  font-size: 1.08rem;
  line-height: 1.3;
}

.lecture-card .langs {
  font-size: 0.82rem;
  color: var(--color-text-soft);
  margin-bottom: 12px;
}

.lecture-card .resources {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
  margin-bottom: 14px;
}

.chip {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  background: var(--color-surface-2);
  color: var(--color-text-soft);
  border-radius: 999px;
  padding: 3px 10px;
  font-size: 0.78rem;
  font-weight: 500;
}

.chip strong {
  color: var(--color-text);
  font-weight: 700;
}

.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  padding: 10px 16px;
  border-radius: var(--radius-md);
  background: var(--color-primary);
  color: #fff;
  font-weight: 600;
  font-size: 0.95rem;
  border: none;
  transition: background var(--transition), transform var(--transition);
}

.btn:hover {
  background: var(--color-primary-hover);
  text-decoration: none;
}

.btn-ghost {
  background: transparent;
  color: var(--color-primary);
  border: 1px solid var(--color-border);
}

.btn-ghost:hover {
  background: var(--color-primary-soft);
}

.lecture-card .btn {
  margin-top: auto;
  align-self: flex-start;
}

.empty-state {
  background: var(--color-surface);
  border: 1px dashed var(--color-border);
  border-radius: var(--radius-md);
  padding: 28px;
  text-align: center;
  color: var(--color-text-soft);
}

/* Footer */
.site-footer {
  margin-top: 40px;
  padding: 24px 0 32px;
  border-top: 1px solid var(--color-border);
  background: var(--color-surface);
  color: var(--color-text-soft);
  font-size: 0.9rem;
}

.site-footer .container {
  display: flex;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: 8px;
}

/* Lecture page */
.lecture-page {
  padding: 32px 0 64px;
}

.breadcrumb {
  margin-bottom: 18px;
  font-size: 0.9rem;
  color: var(--color-text-soft);
}

.breadcrumb a {
  color: var(--color-text-soft);
}

.breadcrumb a:hover {
  color: var(--color-primary);
}

.lecture-title {
  font-size: clamp(1.6rem, 2.6vw, 2.1rem);
  margin: 0 0 8px;
  letter-spacing: -0.01em;
}

.lecture-subtitle {
  margin: 0 0 24px;
  color: var(--color-text-soft);
}

.resource-section {
  margin-top: 36px;
  padding-top: 24px;
  border-top: 1px solid var(--color-border);
}

.resource-section:first-of-type {
  border-top: none;
  padding-top: 0;
  margin-top: 24px;
}

.resource-section h2 {
  margin: 0 0 16px;
  font-size: 1.25rem;
  display: flex;
  align-items: center;
  gap: 10px;
}

.resource-section h2 .count {
  background: var(--color-primary-soft);
  color: var(--color-primary);
  font-size: 0.75rem;
  font-weight: 700;
  padding: 2px 8px;
  border-radius: 999px;
}

.lesson-body {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: 26px 28px;
  color: var(--color-text);
  font-size: 1.02rem;
  line-height: 1.7;
}

.lesson-body p {
  margin: 0 0 14px;
}

.lesson-body p:last-child {
  margin-bottom: 0;
}

.lesson-body h2 {
  margin: 24px 0 12px;
  font-size: 1.35rem;
  letter-spacing: -0.01em;
}

.lesson-body h2:first-child {
  margin-top: 0;
}

.lesson-body h3 {
  margin: 20px 0 10px;
  font-size: 1.15rem;
}

.lesson-body h4 {
  margin: 16px 0 8px;
  font-size: 1.02rem;
  color: var(--color-text-soft);
}

.lesson-body ul,
.lesson-body ol {
  margin: 0 0 14px;
  padding-left: 22px;
}

.lesson-body li {
  margin-bottom: 4px;
}

.lesson-body strong {
  color: var(--color-text);
}

.lesson-body em {
  font-style: italic;
}

.lesson-body a {
  color: var(--color-primary);
  word-break: break-word;
}

.lesson-body a[target="_blank"]::after {
  content: " \2197";
  font-size: 0.85em;
  color: var(--color-text-soft);
}

.lesson-image {
  display: block;
  max-width: 100%;
  height: auto;
  margin: 16px auto;
  border-radius: var(--radius-md);
  box-shadow: var(--color-shadow-soft);
}

.lesson-figure {
  margin: 18px 0;
  text-align: center;
}

.lesson-figure.lesson-video {
  margin: 22px auto;
}

.lesson-figure.lesson-video video {
  width: 100%;
  max-width: 720px;
  border-radius: var(--radius-md);
  box-shadow: var(--color-shadow-soft);
  background: #000;
}

.lesson-caption {
  margin-top: 18px;
  margin-bottom: 6px;
  color: var(--color-text-soft);
  font-style: italic;
  font-size: 0.95rem;
}

.lesson-caption strong {
  color: var(--color-text);
  font-style: normal;
}

.lesson-figure .lesson-image {
  margin: 0 auto;
}

.lesson-body p .lesson-image {
  margin: 12px auto;
}

.lesson-body.is-placeholder {
  background: var(--color-surface-2);
  border-style: dashed;
  color: var(--color-text-soft);
  font-size: 0.95rem;
}

.image-grid {
  display: grid;
  gap: 14px;
  grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
}

.image-grid a {
  display: block;
  border-radius: var(--radius-md);
  overflow: hidden;
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  box-shadow: var(--color-shadow-soft);
  transition: transform var(--transition), box-shadow var(--transition);
}

.image-grid a:hover {
  transform: translateY(-2px);
  box-shadow: var(--color-shadow);
}

.image-grid img {
  width: 100%;
  height: 160px;
  object-fit: cover;
}

.video-grid {
  display: grid;
  gap: 18px;
  grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
}

.video-card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  overflow: hidden;
  box-shadow: var(--color-shadow-soft);
  display: flex;
  flex-direction: column;
}

.video-card h3 {
  margin: 0;
  padding: 12px 14px;
  font-size: 0.98rem;
  border-bottom: 1px solid var(--color-border);
}

.video-card video {
  width: 100%;
  height: auto;
  display: block;
  background: #000;
}

.video-card .unplayable {
  padding: 16px;
  color: var(--color-text-soft);
  font-size: 0.9rem;
}

.video-card .unplayable a {
  display: inline-block;
  margin-top: 6px;
}

.animation-grid,
.simulation-grid {
  display: grid;
  gap: 18px;
  grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
}

.animation-card,
.simulation-card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: 18px;
  display: flex;
  flex-direction: column;
  gap: 10px;
  box-shadow: var(--color-shadow-soft);
}

.animation-card h3,
.simulation-card h3 {
  margin: 0;
  font-size: 1rem;
  display: flex;
  align-items: center;
  gap: 8px;
}

.animation-card .iframe-wrap,
.simulation-card .iframe-wrap {
  position: relative;
  width: 100%;
  padding-bottom: 56.25%;
  border-radius: var(--radius-md);
  overflow: hidden;
  background: var(--color-surface-2);
  border: 1px solid var(--color-border);
}

.animation-card iframe,
.simulation-card iframe {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  border: 0;
}

.simulation-card .badge {
  background: var(--color-accent);
  color: #fff;
  font-size: 0.7rem;
  padding: 2px 8px;
  border-radius: 999px;
  font-weight: 700;
  text-transform: uppercase;
}

.animation-card p,
.simulation-card p {
  margin: 0;
  color: var(--color-text-soft);
  font-size: 0.9rem;
}

.actions-row {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
}

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

@media (max-width: 540px) {
  .container {
    padding: 0 16px;
  }
  .hero {
    padding: 32px 0 16px;
  }
  .image-grid img {
    height: 130px;
  }
  .site-header .container {
    gap: 8px;
  }
  .nav-link {
    display: none;
  }
}