/* ==========================================================
   York 周老师物理 — Shared Styles
   供 index.html 及各子页面套用
   ========================================================== */

/* ---------- CSS Variables (Theme) ---------- */
:root {
  --bg: #f5f8fc;
  --surface: #fff;
  --surface-soft: #edf5ff;
  --ink: #172033;
  --muted: #657184;
  --brand: #0f3a63;
  --brand-2: #1766a6;
  --tech: #23a7e6;
  --line: #dce6f1;
  --shadow: 0 16px 40px rgba(17, 38, 64, 0.10);
  --shadow-soft: 0 8px 24px rgba(17, 38, 64, 0.08);
  --radius: 18px;
  --font: Inter, Poppins, Roboto, "Segoe UI", "PingFang SC", "Microsoft YaHei", sans-serif;
}

/* ---------- Reset & Base ---------- */
* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  font-family: var(--font);
  color: var(--ink);
  background:
    radial-gradient(circle at 15% 0%, rgba(35, 167, 230, 0.12), transparent 28rem),
    linear-gradient(180deg, #fff 0%, var(--bg) 45%, #eef4fb 100%);
  min-height: 100vh;
}

a { color: inherit; text-decoration: none; }

/* ---------- Navigation Shell ---------- */
.site-header {
  position: relative;
  overflow: visible;
  padding-top: 76px;
  background:
    linear-gradient(rgba(15, 58, 99, 0.04) 1px, transparent 1px),
    linear-gradient(90deg, rgba(15, 58, 99, 0.04) 1px, transparent 1px),
    linear-gradient(135deg, #fff 0%, #f1f8ff 100%);
  background-size: 28px 28px, 28px 28px, auto;
  border-bottom: 1px solid var(--line);
}

.container {
  width: min(1180px, calc(100% - 32px));
  margin: 0 auto;
  position: relative;
  z-index: 1;
}

.nav-shell {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  z-index: 1000;
  background: rgba(255, 255, 255, 0.68);
  border-bottom: 1px solid rgba(220, 230, 241, 0.72);
  box-shadow: 0 12px 32px rgba(17, 38, 64, 0.10);
  backdrop-filter: blur(18px) saturate(180%);
  -webkit-backdrop-filter: blur(18px) saturate(180%);
}

.nav {
  min-height: 76px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 24px;
}

/* ---------- Brand ---------- */
.brand {
  display: flex;
  align-items: center;
  gap: 12px;
  font-weight: 800;
  color: var(--brand);
  letter-spacing: 0;
  white-space: nowrap;
}

.brand-mark {
  width: 42px;
  height: 42px;
  display: grid;
  place-items: center;
  border-radius: 14px;
  background: linear-gradient(135deg, var(--brand), var(--tech));
  color: #fff;
  box-shadow: var(--shadow-soft);
  font-size: 20px;
}

.brand small {
  display: block;
  color: var(--muted);
  font-size: 12px;
  font-weight: 600;
  margin-top: 2px;
}

/* ---------- Hamburger Menu Toggle ---------- */
.menu-toggle {
  display: none;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  gap: 5px;
  width: 40px;
  height: 40px;
  padding: 0;
  border: none;
  border-radius: 10px;
  background: transparent;
  cursor: pointer;
  transition: background 0.2s ease;
}

.menu-toggle .bar {
  display: block;
  width: 22px;
  height: 2.5px;
  background: var(--brand);
  border-radius: 2px;
  transition: transform 0.3s ease, opacity 0.3s ease;
}

.menu-toggle:hover { background: rgba(15, 58, 99, 0.06); }

.menu-toggle.is-active .bar:nth-child(1) {
  transform: translateY(7.5px) rotate(45deg);
}

.menu-toggle.is-active .bar:nth-child(2) {
  opacity: 0;
}

.menu-toggle.is-active .bar:nth-child(3) {
  transform: translateY(-7.5px) rotate(-45deg);
}

/* ---------- Category Menu & Dropdowns ---------- */
.category-menu {
  display: flex;
  align-items: center;
  justify-content: flex-end;
  gap: 8px;
  color: #344155;
  font-size: 14px;
  font-weight: 700;
  flex-wrap: wrap;
}

.menu-item { position: relative; }

.menu-top {
  display: block;
  padding: 10px 14px;
  border-radius: 999px;
  transition: background 0.2s ease, color 0.2s ease;
}

.menu-item:hover .menu-top,
.menu-top:focus {
  background: #e8f4ff;
  color: var(--brand-2);
}

.submenu {
  position: absolute;
  top: calc(100% + 8px);
  left: 0;
  min-width: 230px;
  padding: 8px;
  border: 1px solid var(--line);
  border-radius: 14px;
  background: #fff;
  box-shadow: var(--shadow);
  opacity: 0;
  visibility: hidden;
  transform: translateY(6px);
  transition: opacity 0.18s ease, transform 0.18s ease, visibility 0.18s ease;
  z-index: 20;
}

.menu-item:hover .submenu,
.menu-item:focus-within .submenu {
  opacity: 1;
  visibility: visible;
  transform: translateY(0);
}

.submenu a {
  display: block;
  padding: 10px 12px;
  border-radius: 10px;
  color: #344155;
  font-size: 13px;
  line-height: 1.4;
  transition: background 0.2s ease, color 0.2s ease;
}

.submenu a:hover {
  background: #eef8ff;
  color: var(--brand-2);
}

/* ---------- Hero Section ---------- */
.hero {
  padding: 58px 0 46px;
  display: grid;
  grid-template-columns: minmax(0, 1.25fr) 0.75fr;
  align-items: center;
  gap: 42px;
}

.eyebrow {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  color: var(--brand-2);
  background: rgba(35, 167, 230, 0.10);
  border: 1px solid rgba(35, 167, 230, 0.22);
  padding: 8px 12px;
  border-radius: 999px;
  font-size: 13px;
  font-weight: 800;
}

h1 {
  margin: 18px 0 16px;
  font-size: clamp(34px, 5vw, 62px);
  line-height: 1.06;
  color: var(--brand);
  letter-spacing: 0;
}

.hero-text {
  max-width: 760px;
  color: var(--muted);
  font-size: 18px;
  line-height: 1.8;
  margin: 0;
}

.hero-panel {
  background: rgba(255, 255, 255, 0.78);
  border: 1px solid var(--line);
  border-radius: 24px;
  padding: 22px;
  box-shadow: var(--shadow);
  backdrop-filter: blur(10px);
}

/* ---------- Search & Filter Panel ---------- */
.search-box { display: grid; gap: 14px; }

.search-box input,
.search-box select {
  width: 100%;
  border: 1px solid var(--line);
  border-radius: 14px;
  padding: 13px 14px;
  color: var(--ink);
  background: #fff;
  font: inherit;
  outline: none;
  transition: border-color 0.2s ease, box-shadow 0.2s ease;
}

.search-box input:focus,
.search-box select:focus {
  border-color: var(--tech);
  box-shadow: 0 0 0 4px rgba(35, 167, 230, 0.14);
}

.filter-row {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 10px;
}

/* ---------- Topic Sections & Experiment Grid ---------- */
.section {
  padding: 16px 0 26px;
  scroll-margin-top: 96px;
}

.section-heading {
  display: flex;
  align-items: flex-end;
  justify-content: space-between;
  gap: 18px;
  margin: 18px 0;
}

.section-heading h2 {
  margin: 0;
  color: var(--brand);
  font-size: 24px;
}

.section-heading p {
  margin: 0;
  color: var(--muted);
  font-size: 14px;
}

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

/* ---------- Experiment Cards ---------- */
.experiment-card,
.empty-card {
  background: var(--surface);
  border: 1px solid var(--line);
  box-shadow: var(--shadow-soft);
  border-radius: 18px;
  padding: 20px;
}

.experiment-card {
  position: relative;
  min-height: 212px;
  display: flex;
  flex-direction: column;
  overflow: hidden;
  scroll-margin-top: 108px;
  transition: transform 0.22s ease, box-shadow 0.22s ease, border-color 0.22s ease;
}

.experiment-card::before {
  content: "";
  position: absolute;
  inset: 0 0 auto;
  height: 4px;
  background: linear-gradient(90deg, var(--brand-2), var(--tech));
  opacity: 0.9;
}

.experiment-card:hover {
  transform: translateY(-6px);
  border-color: rgba(35, 167, 230, 0.55);
  box-shadow: var(--shadow);
}

.experiment-card h3 {
  margin: 8px 0 10px;
  color: var(--ink);
  font-size: 18px;
  line-height: 1.35;
}

.experiment-card p {
  margin: 0;
  color: var(--muted);
  font-size: 14px;
  line-height: 1.7;
}

.difficulty {
  align-self: flex-start;
  color: #38506a;
  background: #f2f7fd;
  border: 1px solid var(--line);
  border-radius: 999px;
  padding: 6px 10px;
  margin-bottom: 12px;
  font-size: 12px;
  font-weight: 800;
}

.tags {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin-top: 18px;
}

.tag {
  color: var(--brand-2);
  background: rgba(35, 167, 230, 0.10);
  border: 1px solid rgba(35, 167, 230, 0.18);
  border-radius: 999px;
  padding: 5px 9px;
  font-size: 12px;
  font-weight: 800;
}

/* ---------- Empty Cards ---------- */
.empty-card {
  min-height: 140px;
  color: var(--muted);
  line-height: 1.7;
  background: #f9fcff;
}

/* ---------- Quick Nav ---------- */
.quick-nav {
  position: fixed;
  right: 20px;
  bottom: 24px;
  z-index: 20;
  display: grid;
  gap: 10px;
}

.quick-nav a,
.quick-nav button {
  border: 1px solid var(--line);
  background: rgba(255, 255, 255, 0.92);
  color: var(--brand);
  box-shadow: var(--shadow-soft);
  width: 46px;
  height: 46px;
  border-radius: 16px;
  display: grid;
  place-items: center;
  cursor: pointer;
  font: inherit;
  font-weight: 900;
  transition: transform 0.2s ease, background 0.2s ease;
}

.quick-nav a:hover,
.quick-nav button:hover {
  transform: translateY(-2px);
  background: #eef8ff;
}

/* ---------- Footer ---------- */
.footer {
  border-top: 1px solid var(--line);
  background: #fff;
  padding: 32px 0;
  color: var(--muted);
  font-size: 14px;
}

.footer-inner {
  display: grid;
  grid-template-columns: 1.2fr 1fr 1fr;
  gap: 24px;
}

.footer strong {
  display: block;
  color: var(--brand);
  margin-bottom: 8px;
}

.footer p {
  margin: 6px 0;
  line-height: 1.7;
}

/* ==========================================================
   Responsive — Tablet (≤960px)
   ========================================================== */
@media (max-width: 960px) {
  .hero,
  .footer-inner {
    grid-template-columns: 1fr;
  }

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

/* ==========================================================
   Responsive — Mobile (≤680px)
   ========================================================== */
@media (max-width: 680px) {
  .nav {
    align-items: flex-start;
    flex-direction: column;
    padding: 16px 0;
  }

  /* Show hamburger, hide category menu by default */
  .menu-toggle {
    display: flex;
    position: absolute;
    top: 18px;
    right: 0;
  }

  .nav {
    position: relative;
  }

  .category-menu {
    display: none;
    width: 100%;
    flex-direction: column;
    align-items: stretch;
    gap: 4px;
    padding-top: 12px;
    overflow: visible;
    flex-wrap: nowrap;
  }

  .category-menu.nav-open {
    display: flex;
  }

  .menu-item {
    width: 100%;
  }

  .menu-top {
    width: 100%;
    text-align: left;
    padding: 12px 14px;
    border-radius: 12px;
  }

  /* Touch-friendly submenu on mobile */
  .submenu {
    position: static;
    min-width: auto;
    padding: 4px 4px 4px 16px;
    border: none;
    border-radius: 0;
    background: transparent;
    box-shadow: none;
    opacity: 1;
    visibility: visible;
    transform: none;
    transition: none;
    max-height: 0;
    overflow: hidden;
    transition: max-height 0.3s ease;
  }

  .submenu.open {
    max-height: 400px;
  }

  .menu-item:hover .submenu {
    opacity: 1;
    visibility: visible;
    transform: none;
  }

  .submenu a {
    border-radius: 8px;
    padding: 10px 12px;
    font-size: 13px;
  }

  .hero {
    padding: 32px 0;
  }

  .filter-row,
  .experiment-grid {
    grid-template-columns: 1fr;
  }

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

  .quick-nav {
    right: 12px;
    bottom: 12px;
  }

  .quick-nav a,
  .quick-nav button {
    width: 40px;
    height: 40px;
    border-radius: 12px;
  }
}

/* ==========================================================
   Responsive — Small Phone (≤480px)
   ========================================================== */
@media (max-width: 480px) {
  .container {
    width: calc(100% - 20px);
  }

  .brand {
    gap: 8px;
    font-size: 15px;
  }

  .brand-mark {
    width: 36px;
    height: 36px;
    font-size: 16px;
    border-radius: 10px;
  }

  .brand small {
    font-size: 10px;
  }

  .menu-toggle {
    top: 15px;
    width: 36px;
    height: 36px;
  }

  .menu-toggle .bar {
    width: 18px;
    height: 2px;
  }

  .menu-toggle.is-active .bar:nth-child(1) {
    transform: translateY(6px) rotate(45deg);
  }

  .menu-toggle.is-active .bar:nth-child(3) {
    transform: translateY(-6px) rotate(-45deg);
  }

  h1 {
    font-size: 28px;
  }

  .hero-text {
    font-size: 16px;
  }

  .hero-panel {
    padding: 16px;
    border-radius: 18px;
  }

  .eyebrow {
    font-size: 12px;
    padding: 6px 10px;
  }

  .section-heading h2 {
    font-size: 20px;
  }

  .experiment-card {
    min-height: 180px;
    padding: 16px;
  }

  .experiment-card h3 {
    font-size: 16px;
  }

  .footer-inner {
    gap: 16px;
  }

  .footer {
    padding: 24px 0;
    font-size: 13px;
  }
}