/* ============================================================
   PaperVault — Liquid Glass Design System
   Premium glassmorphism, responsive, desktop + mobile.
   ============================================================ */

@import url('https://fonts.googleapis.com/css2?family=Fraunces:opsz,wght@9..144,400..700&family=Manrope:wght@400;500;600;700;800&display=swap');

:root {
  --bg-0: #070b14;
  --bg-1: #0b1322;
  --glass: rgba(255, 255, 255, 0.06);
  --glass-strong: rgba(255, 255, 255, 0.10);
  --glass-border: rgba(255, 255, 255, 0.14);
  --glass-border-hi: rgba(255, 255, 255, 0.28);
  --txt: #eaf0fb;
  --txt-dim: #9fb0cc;
  --txt-faint: #6b7d9e;

  /* subject accent ramps */
  --maths: #5ee0c8;
  --maths-2: #2bb3ff;
  --cs: #b78bff;
  --cs-2: #ff7bd0;

  --ok: #4ade80;
  --radius: 22px;
  --radius-sm: 14px;
  --shadow: 0 20px 60px -20px rgba(0,0,0,.65);
  --blur: blur(26px) saturate(160%);
  --font-display: 'Fraunces', Georgia, serif;
  --font-body: 'Manrope', system-ui, sans-serif;
}

* { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; }
body {
  font-family: var(--font-body);
  color: var(--txt);
  background: var(--bg-0);
  min-height: 100vh;
  line-height: 1.55;
  overflow-x: hidden;
  -webkit-font-smoothing: antialiased;
}

/* ---- Animated aurora background ---- */
.aurora {
  position: fixed; inset: 0; z-index: -2;
  background:
    radial-gradient(60% 50% at 15% 10%, rgba(43,179,255,.22), transparent 60%),
    radial-gradient(50% 50% at 85% 20%, rgba(183,139,255,.20), transparent 60%),
    radial-gradient(60% 60% at 70% 90%, rgba(94,224,200,.16), transparent 60%),
    linear-gradient(180deg, var(--bg-0), var(--bg-1));
}
.aurora::after {
  content: ''; position: absolute; inset: -40%;
  background: conic-gradient(from 0deg at 50% 50%,
    transparent, rgba(91,224,200,.06), transparent, rgba(183,139,255,.06), transparent);
  animation: spin 40s linear infinite;
}
@keyframes spin { to { transform: rotate(360deg); } }
.grain {
  position: fixed; inset: 0; z-index: -1; pointer-events: none; opacity: .04;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='120' height='120'%3E%3Cfilter id='n'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='.9' numOctaves='3'/%3E%3C/filter%3E%3Crect width='100%25' height='100%25' filter='url(%23n)'/%3E%3C/svg%3E");
}

/* ---- Glass primitives ---- */
.glass {
  background: var(--glass);
  border: 1px solid var(--glass-border);
  -webkit-backdrop-filter: var(--blur);
  backdrop-filter: var(--blur);
  border-radius: var(--radius);
  box-shadow: var(--shadow), inset 0 1px 0 rgba(255,255,255,.10);
}
.glass-hover { transition: transform .35s cubic-bezier(.2,.8,.2,1), border-color .35s, box-shadow .35s; }
.glass-hover:hover {
  transform: translateY(-4px);
  border-color: var(--glass-border-hi);
  box-shadow: 0 30px 70px -24px rgba(0,0,0,.7), inset 0 1px 0 rgba(255,255,255,.18);
}

.wrap { max-width: 1180px; margin: 0 auto; padding: 0 22px; }

/* ---- Top bar ---- */
.nav {
  position: sticky; top: 0; z-index: 50;
  display: flex; align-items: center; justify-content: space-between;
  padding: 14px 22px; margin: 14px auto; max-width: 1180px;
}
.brand { display: flex; align-items: center; gap: 12px; font-weight: 800; letter-spacing: -.02em; font-size: 1.15rem; text-decoration: none; color: var(--txt); }
.brand .dot { width: 30px; height: 30px; border-radius: 9px; background: linear-gradient(135deg, var(--maths), var(--cs)); box-shadow: 0 0 24px rgba(94,224,200,.5); }
.nav-links { display: flex; gap: 8px; }
.nav-links a {
  color: var(--txt-dim); text-decoration: none; font-weight: 600; font-size: .92rem;
  padding: 8px 14px; border-radius: 11px; transition: .25s;
}
.nav-links a:hover, .nav-links a.active { color: var(--txt); background: var(--glass-strong); }

/* ---- Hero ---- */
.hero { padding: 64px 0 30px; text-align: center; }
.hero h1 {
  font-family: var(--font-display); font-weight: 600; letter-spacing: -.03em;
  font-size: clamp(2.6rem, 6vw, 4.6rem); line-height: 1.02;
  background: linear-gradient(120deg, #fff 20%, var(--maths) 55%, var(--cs-2) 95%);
  -webkit-background-clip: text; background-clip: text; color: transparent;
}
.hero p { color: var(--txt-dim); max-width: 600px; margin: 18px auto 0; font-size: 1.08rem; }
.kicker {
  display: inline-flex; align-items: center; gap: 8px; font-size: .8rem; font-weight: 700;
  letter-spacing: .14em; text-transform: uppercase; color: var(--txt-faint);
  padding: 7px 14px; border-radius: 999px; border: 1px solid var(--glass-border);
  background: var(--glass); margin-bottom: 22px;
}

/* ---- Subject cards grid ---- */
.grid { display: grid; gap: 18px; }
.grid-2 { grid-template-columns: repeat(2, 1fr); }
.grid-3 { grid-template-columns: repeat(3, 1fr); }
.subjects { margin: 30px 0 60px; }

.subject-card { padding: 26px; position: relative; overflow: hidden; text-decoration: none; color: var(--txt); display: block; }
.subject-card .ribbon {
  position: absolute; top: 0; left: 0; right: 0; height: 4px;
  background: linear-gradient(90deg, var(--a1), var(--a2));
}
.subject-card .code { font-size: .8rem; font-weight: 700; color: var(--txt-faint); letter-spacing: .08em; }
.subject-card h3 { font-family: var(--font-display); font-size: 1.7rem; font-weight: 600; margin: 8px 0 4px; }
.subject-card .comp { color: var(--txt-dim); font-size: .95rem; }
.subject-card .meta { display: flex; gap: 16px; margin-top: 18px; font-size: .85rem; color: var(--txt-faint); }
.subject-card .meta b { color: var(--txt); font-weight: 700; }
.subject-card .glow {
  position: absolute; width: 220px; height: 220px; border-radius: 50%; right: -60px; bottom: -80px;
  background: radial-gradient(circle, var(--a1), transparent 70%); opacity: .25; filter: blur(10px);
}
.sc-maths { --a1: var(--maths); --a2: var(--maths-2); }
.sc-cs    { --a1: var(--cs);    --a2: var(--cs-2); }

/* ---- Topic list ---- */
.crumbs { display: flex; gap: 8px; align-items: center; color: var(--txt-faint); font-size: .88rem; margin: 8px 0 22px; }
.crumbs a { color: var(--txt-dim); text-decoration: none; }
.crumbs a:hover { color: var(--txt); }

.topic-row {
  display: flex; align-items: center; gap: 16px; padding: 18px 22px; text-decoration: none; color: var(--txt);
  border-radius: var(--radius-sm); margin-bottom: 10px;
}
.topic-row .tcode {
  flex-shrink: 0; width: 46px; height: 46px; border-radius: 12px; display: grid; place-items: center;
  font-weight: 800; font-size: .9rem; background: var(--glass-strong); border: 1px solid var(--glass-border); color: var(--txt-dim);
}
.topic-row .ttitle { font-weight: 700; font-size: 1.08rem; }
.topic-row .tcount { margin-left: auto; font-size: .82rem; color: var(--txt-faint); white-space: nowrap; }
.topic-row .chev { color: var(--txt-faint); transition: transform .25s; }
.topic-row:hover .chev { transform: translateX(4px); color: var(--txt); }

/* ---- Filter chips ---- */
.filters { display: flex; flex-wrap: wrap; gap: 8px; margin: 6px 0 24px; }
.chip {
  padding: 8px 16px; border-radius: 999px; border: 1px solid var(--glass-border);
  background: var(--glass); color: var(--txt-dim); font-weight: 600; font-size: .86rem;
  cursor: pointer; text-decoration: none; transition: .25s;
}
.chip:hover { color: var(--txt); border-color: var(--glass-border-hi); }
.chip.active { background: linear-gradient(120deg, var(--maths), var(--cs)); color: #06121f; border-color: transparent; }

/* ---- Question card + reveal ---- */
.q-card { padding: 0; margin-bottom: 16px; overflow: hidden; }
.q-head { display: flex; align-items: center; gap: 14px; padding: 18px 22px; }
.q-num { font-weight: 800; font-size: 1.05rem; }
.q-tags { display: flex; gap: 8px; flex-wrap: wrap; margin-left: auto; }
.q-tag { font-size: .76rem; font-weight: 700; padding: 5px 11px; border-radius: 999px; background: var(--glass-strong); color: var(--txt-dim); border: 1px solid var(--glass-border); }
.q-body { padding: 0 22px 18px; }
.q-frame { width: 100%; border: 0; border-radius: var(--radius-sm); background: #fff; min-height: 360px; }
.q-img { width: 100%; border-radius: var(--radius-sm); display: block; background:#fff; }

.btn {
  display: inline-flex; align-items: center; gap: 9px; cursor: pointer;
  padding: 11px 18px; border-radius: 12px; font-weight: 700; font-size: .9rem;
  border: 1px solid var(--glass-border); background: var(--glass-strong); color: var(--txt);
  text-decoration: none; transition: .25s;
}
.btn:hover { border-color: var(--glass-border-hi); transform: translateY(-1px); }
.btn-accent { background: linear-gradient(120deg, var(--maths), var(--maths-2)); color: #06121f; border-color: transparent; }
.btn-row { display: flex; gap: 10px; flex-wrap: wrap; padding: 0 22px 20px; }

.reveal { max-height: 0; overflow: hidden; transition: max-height .5s cubic-bezier(.2,.8,.2,1); }
.reveal.open { max-height: 2400px; }
.scheme-wrap { margin: 0 22px 22px; padding: 18px; border-radius: var(--radius-sm); background: rgba(74,222,128,.06); border: 1px solid rgba(74,222,128,.22); }
.scheme-wrap .label { font-size: .78rem; font-weight: 800; letter-spacing: .1em; text-transform: uppercase; color: var(--ok); margin-bottom: 12px; }

/* ---- Empty / notices ---- */
.empty { text-align: center; padding: 70px 24px; color: var(--txt-dim); }
.empty .em-ico { font-size: 2.4rem; margin-bottom: 12px; opacity: .7; }
.notice { padding: 16px 20px; border-radius: var(--radius-sm); font-size: .9rem; }
.notice-warn { background: rgba(255,196,77,.08); border: 1px solid rgba(255,196,77,.28); color: #ffd98a; }

/* ---- Revision prose ---- */
.prose { padding: 30px; }
.prose h2 { font-family: var(--font-display); font-weight: 600; margin: 22px 0 10px; font-size: 1.5rem; }
.prose p { color: var(--txt-dim); margin-bottom: 12px; }
.prose ul { color: var(--txt-dim); padding-left: 22px; margin-bottom: 12px; }
.prose .src { margin-top: 24px; font-size: .82rem; color: var(--txt-faint); border-top: 1px solid var(--glass-border); padding-top: 14px; }
.prose .src a { color: var(--maths); }

/* ---- Footer ---- */
.foot { text-align: center; padding: 50px 22px 40px; color: var(--txt-faint); font-size: .82rem; }
.foot .legal { max-width: 640px; margin: 14px auto 0; line-height: 1.7; }

/* ---- Mobile menu ---- */
.menu-btn { display: none; background: var(--glass); border: 1px solid var(--glass-border); border-radius: 11px; padding: 9px 12px; color: var(--txt); cursor: pointer; }

@media (max-width: 860px) {
  .grid-2, .grid-3 { grid-template-columns: 1fr; }
  .nav-links { position: fixed; inset: 0 0 auto 0; top: 70px; flex-direction: column; margin: 12px; padding: 12px;
    background: rgba(11,19,34,.92); border: 1px solid var(--glass-border); border-radius: var(--radius);
    -webkit-backdrop-filter: var(--blur); backdrop-filter: var(--blur); display: none; }
  .nav-links.show { display: flex; }
  .menu-btn { display: block; }
  .hero { padding: 40px 0 20px; }
  .q-head, .btn-row, .q-body { padding-left: 16px; padding-right: 16px; }
}

/* staggered entrance */
.rise { opacity: 0; transform: translateY(16px); animation: rise .7s cubic-bezier(.2,.8,.2,1) forwards; }
@keyframes rise { to { opacity: 1; transform: none; } }