/* AI English Tutor — neural cyan + violet study layout */
:root {
  --ink: #0e1424;
  --ink-soft: #34405d;
  --muted: #6b7896;
  --paper: #f5f8ff;
  --paper-2: #eaf2ff;
  --surface: #ffffff;
  --line: #d8e2f4;
  --line-strong: #b8c8e6;
  --cyan: #00b4d8;
  --cyan-deep: #0090b1;
  --violet: #7466a8;
  --violet-deep: #4a3f80;
  --indigo: #1f2a4d;
  --pink: #d946ef;
  --amber: #f59e0b;
  --teal: #14b8a6;
  --code-bg: #0c1224;
  --code-text: #e7eeff;
  --mono: 'SF Mono', 'JetBrains Mono', 'Fira Code', Consolas, monospace;
  --display: 'Songti SC', 'STSong', 'Noto Serif CJK SC', 'Iowan Old Style', serif;
  --sans: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'PingFang SC', 'Noto Sans CJK SC', sans-serif;
  --radius: 10px;
}
* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  font-family: var(--sans);
  color: var(--ink);
  background:
    radial-gradient(ellipse at 12% 8%, rgba(0,180,216,.10), transparent 55%),
    radial-gradient(ellipse at 88% 24%, rgba(116,102,168,.12), transparent 60%),
    linear-gradient(90deg, rgba(31,42,77,.05) 1px, transparent 1px),
    linear-gradient(0deg, rgba(31,42,77,.045) 1px, transparent 1px),
    var(--paper);
  background-size: auto, auto, 32px 32px, 32px 32px, auto;
  line-height: 1.78;
  -webkit-font-smoothing: antialiased;
  overflow-x: hidden;
}
.nav {
  position: sticky;
  top: 0;
  z-index: 100;
  min-height: 54px;
  padding: 0 24px;
  display: flex;
  align-items: center;
  gap: 16px;
  background: rgba(245,248,255,.92);
  border-bottom: 1px solid var(--line);
  backdrop-filter: blur(16px);
}
.nav-back { color: var(--violet-deep); text-decoration: none; font-size: .88rem; font-weight: 700; }
.nav-back::before { content: '‹'; margin-right: 4px; font-size: 1.2rem; }
.nav-title { flex: 1; min-width: 0; color: var(--ink-soft); font-size: .92rem; font-weight: 700; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
.nav-badge { color: #fff; background: linear-gradient(135deg, var(--cyan), var(--violet)); border-radius: 999px; padding: 3px 10px; font-size: .68rem; font-weight: 800; letter-spacing: .06em; }
.hero {
  color: #fff;
  background:
    radial-gradient(circle at 18% 22%, rgba(0,180,216,.45), transparent 45%),
    radial-gradient(circle at 82% 68%, rgba(217,70,239,.25), transparent 50%),
    linear-gradient(135deg, #0c1224 0%, #1f2a4d 55%, #2a1f4d 100%),
    repeating-linear-gradient(90deg, rgba(255,255,255,.05) 0 1px, transparent 1px 72px);
  padding: 74px 24px 58px;
  overflow: hidden;
  position: relative;
}
.hero::before {
  content: '';
  position: absolute;
  inset: 0;
  background-image:
    linear-gradient(0deg, rgba(0,180,216,.10) 1px, transparent 1px),
    linear-gradient(90deg, rgba(0,180,216,.10) 1px, transparent 1px);
  background-size: 48px 48px;
  mask-image: radial-gradient(ellipse at 50% 50%, #000 30%, transparent 80%);
  pointer-events: none;
}
.hero-grid { max-width: 1060px; margin: 0 auto; display: grid; grid-template-columns: minmax(0, 1.08fr) minmax(280px, .92fr); gap: 34px; align-items: center; position: relative; z-index: 1; }
.hero-eyebrow, .section-label, .ch-num, .nav-dir { font-family: var(--mono); text-transform: uppercase; letter-spacing: .16em; }
.hero-eyebrow { color: #5be8ff; font-size: .72rem; margin-bottom: 12px; }
.hero h1 { font-family: var(--display); font-size: clamp(2.35rem, 6vw, 4.5rem); line-height: 1.04; margin: 0 0 18px; font-weight: 800; letter-spacing: 0; background: linear-gradient(135deg, #fff 0%, #5be8ff 60%, #d8c5ff 100%); -webkit-background-clip: text; -webkit-text-fill-color: transparent; background-clip: text; }
.hero-sub { max-width: 620px; margin: 0 0 24px; color: rgba(255,255,255,.78); font-size: 1rem; line-height: 1.85; }
.hero-chips { display: flex; flex-wrap: wrap; gap: 8px; }
.chip { border: 1px solid rgba(91,232,255,.32); background: rgba(91,232,255,.10); border-radius: 999px; padding: 5px 12px; font-size: .76rem; color: rgba(255,255,255,.88); }
.tutor-board { border: 1px solid rgba(91,232,255,.28); background: linear-gradient(160deg, rgba(0,180,216,.18), rgba(116,102,168,.18)); border-radius: var(--radius); padding: 22px; box-shadow: 0 22px 70px rgba(0,0,0,.35); position: relative; }
.tutor-board::before { content: 'AI · TUTOR'; position: absolute; top: 12px; right: 14px; font-family: var(--mono); font-size: .62rem; letter-spacing: .14em; color: rgba(91,232,255,.7); }
.chat-line { display: flex; gap: 8px; margin: 10px 0; align-items: flex-start; }
.chat-line .who { font-family: var(--mono); font-size: .68rem; min-width: 42px; padding-top: 2px; }
.chat-line.user .who { color: #fbbf24; }
.chat-line.ai .who { color: #5be8ff; }
.chat-line .msg { background: rgba(0,0,0,.28); border: 1px solid rgba(255,255,255,.08); border-radius: 6px; padding: 8px 12px; color: rgba(255,255,255,.88); font-size: .82rem; line-height: 1.55; flex: 1; }
.chat-line.ai .msg { background: rgba(0,180,216,.16); border-color: rgba(91,232,255,.22); }
.chat-line .msg b { color: #ffd97a; }
.board-tags { display: flex; gap: 8px; flex-wrap: wrap; margin-top: 14px; }
.board-tags span { font-size: .72rem; color: rgba(255,255,255,.72); border-bottom: 1px solid rgba(91,232,255,.4); padding-bottom: 1px; }
.container, .content { width: 100%; max-width: 900px; margin: 0 auto; padding: 38px 24px 84px; }
.content { max-width: 820px; }
.content > * { max-width: 100%; }
.section-label { color: var(--muted); font-size: .68rem; font-weight: 900; margin: 32px 0 10px 2px; }
.card, .feature, .chapter-nav a { background: rgba(255,255,255,.92); border: 1px solid var(--line); border-radius: var(--radius); box-shadow: 0 1px 0 rgba(255,255,255,.9) inset, 0 12px 28px rgba(31,42,77,.06); }
.card { padding: 22px; margin-bottom: 16px; }
.intro-card p:last-child, .prose p:last-child { margin-bottom: 0; }
p { color: var(--ink-soft); margin: 0 0 15px; font-size: .95rem; }
.content p, .content li, .chapter-hero p { overflow-wrap: anywhere; word-break: break-word; }
.prose h2 { margin: 0 0 12px; font-size: 1.18rem; color: var(--violet-deep); }
.prose ol { margin-bottom: 0; }
strong { color: var(--ink); }
.feature-grid { display: grid; grid-template-columns: repeat(4, minmax(0, 1fr)); gap: 12px; }
.feature { padding: 18px; }
.feature span { font-family: var(--mono); color: var(--cyan-deep); font-weight: 900; font-size: .72rem; }
.feature h3 { margin: 7px 0 6px; font-size: 1rem; color: var(--indigo); }
.feature p { margin: 0; color: var(--muted); font-size: .82rem; line-height: 1.65; }
.list-card { padding: 0; overflow: hidden; }
.card-list { list-style: none; margin: 0; padding: 0; }
.card-list li { border-bottom: 1px solid var(--line); }
.card-list li:last-child { border-bottom: 0; }
.card-list a { display: flex; gap: 14px; align-items: flex-start; padding: 15px 16px; color: inherit; text-decoration: none; transition: background .16s, transform .16s; }
.card-list a:hover { background: var(--paper-2); }
.list-num { width: 34px; height: 34px; border-radius: 50%; display: grid; place-items: center; flex: 0 0 auto; background: linear-gradient(135deg, var(--cyan), var(--violet)); color: #fff; font-family: var(--mono); font-size: .72rem; font-weight: 900; box-shadow: 0 4px 14px rgba(0,180,216,.32); }
.list-content { flex: 1; min-width: 0; }
.list-title { color: var(--ink); font-weight: 800; font-size: .96rem; margin-bottom: 2px; }
.list-desc { color: var(--muted); font-size: .8rem; line-height: 1.55; }
.list-chevron { color: var(--line-strong); font-size: 1.2rem; align-self: center; }
.chapter-hero {
  color: #fff;
  text-align: center;
  padding: 54px 24px 46px;
  overflow: hidden;
  position: relative;
  background:
    radial-gradient(circle at 22% 30%, rgba(0,180,216,.4), transparent 55%),
    radial-gradient(circle at 78% 80%, rgba(217,70,239,.25), transparent 50%),
    linear-gradient(135deg, #0c1224 0%, #1f2a4d 60%, #2a1f4d 100%);
}
.chapter-hero::before {
  content: '';
  position: absolute;
  inset: 0;
  background-image:
    linear-gradient(0deg, rgba(91,232,255,.08) 1px, transparent 1px),
    linear-gradient(90deg, rgba(91,232,255,.08) 1px, transparent 1px);
  background-size: 40px 40px;
  mask-image: radial-gradient(ellipse at 50% 50%, #000 30%, transparent 80%);
  pointer-events: none;
}
.chapter-hero > * { position: relative; z-index: 1; }
.chapter-hero .ch-num { color: #5be8ff; font-size: .68rem; margin-bottom: 10px; }
.chapter-hero h1 { font-family: var(--display); font-size: clamp(1.9rem, 5vw, 3rem); line-height: 1.12; margin: 0 auto 12px; letter-spacing: 0; max-width: 760px; overflow-wrap: anywhere; background: linear-gradient(135deg, #fff, #cfe7ff 60%, #d8c5ff); -webkit-background-clip: text; -webkit-text-fill-color: transparent; background-clip: text; }
.chapter-hero h1 br { display: none; }
.chapter-hero p { color: rgba(255,255,255,.78); max-width: 620px; margin: 0 auto; overflow-wrap: anywhere; }
.progress { height: 5px; background: var(--line); border-radius: 99px; margin-bottom: 34px; overflow: hidden; }
.progress-bar { height: 100%; background: linear-gradient(90deg, var(--cyan), var(--violet), var(--pink)); border-radius: inherit; }
.content h2 { font-family: var(--display); color: var(--ink); font-size: 1.52rem; line-height: 1.28; margin: 42px 0 14px; padding-bottom: 10px; border-bottom: 2px solid var(--line); letter-spacing: 0; position: relative; }
.content h2::before { content: ''; position: absolute; left: 0; bottom: -2px; width: 56px; height: 2px; background: linear-gradient(90deg, var(--cyan), var(--violet)); }
.content h2:first-child { margin-top: 0; }
.content h3 { color: var(--violet-deep); font-size: 1.08rem; margin: 28px 0 10px; }
.content h4 { color: var(--cyan-deep); font-size: .76rem; margin: 22px 0 8px; letter-spacing: .08em; text-transform: uppercase; }
.content ul, .content ol { color: var(--ink-soft); padding-left: 24px; margin: 10px 0 16px; font-size: .93rem; }
.content li { margin: 4px 0; }
pre { background: var(--code-bg); color: var(--code-text); border-radius: var(--radius); padding: 17px 18px; overflow-x: auto; font-size: .86rem; line-height: 1.72; margin: 16px 0; border: 1px solid rgba(91,232,255,.18); white-space: pre-wrap; overflow-wrap: anywhere; box-shadow: 0 6px 22px rgba(12,18,36,.18); position: relative; }
pre::before { content: ''; position: absolute; inset: 0 0 auto 0; height: 2px; background: linear-gradient(90deg, var(--cyan), var(--violet), var(--pink)); border-radius: var(--radius) var(--radius) 0 0; opacity: .9; }
code { font-family: var(--mono); }
p code, li code, td code { background: #e7f3fb; color: var(--cyan-deep); border: 1px solid #bce0f0; border-radius: 5px; padding: 1px 5px; font-size: .86em; }
.table-wrap { overflow-x: auto; margin: 18px 0; border: 1px solid var(--line); border-radius: var(--radius); background: var(--surface); }
table { width: 100%; border-collapse: collapse; font-size: .86rem; min-width: 560px; }
th { background: var(--paper-2); color: var(--ink); font-size: .76rem; text-transform: uppercase; letter-spacing: .06em; text-align: left; padding: 11px 13px; border-bottom: 1px solid var(--line-strong); }
td { color: var(--ink-soft); padding: 11px 13px; border-top: 1px solid var(--line); vertical-align: top; }
tr:first-child td { border-top: 0; }
.callout { margin: 18px 0; padding: 15px 16px; border-radius: var(--radius); background: #ecf6fb; border-left: 4px solid var(--cyan); }
.callout-mark { display: block; color: var(--cyan-deep); font-size: .76rem; font-weight: 900; margin-bottom: 4px; letter-spacing: .08em; }
.callout p { margin: 0; }
.callout.warn { background: #fdf2e2; border-left-color: var(--amber); }
.callout.warn .callout-mark { color: #b27510; }
.callout.danger { background: #fbe9eb; border-left-color: #e63946; }
.callout.danger .callout-mark { color: #b81f2c; }
.callout.tip { background: #f1ecfa; border-left-color: var(--violet); }
.callout.tip .callout-mark { color: var(--violet-deep); }
.chapter-nav { display: grid; grid-template-columns: 1fr 1fr; gap: 12px; margin-top: 48px; padding-top: 22px; border-top: 1px solid var(--line); }
.chapter-nav a { display: flex; flex-direction: column; padding: 14px; text-decoration: none; color: inherit; min-height: 80px; justify-content: center; }
.chapter-nav a:hover { border-color: var(--cyan); background: #fff; box-shadow: 0 12px 30px rgba(0,180,216,.12); }
.next { text-align: right; }
.nav-dir { color: var(--muted); font-size: .66rem; font-weight: 900; }
.nav-title-text { color: var(--violet-deep); font-weight: 800; font-size: .88rem; line-height: 1.45; }
.site-footer { border-top: 1px solid var(--line); background: rgba(234,242,255,.82); padding: 28px 24px; text-align: center; color: var(--muted); font-size: .82rem; }
.site-footer a { color: var(--violet-deep); text-decoration: none; font-weight: 700; }
@media (max-width: 760px) {
  .nav { padding: 0 16px; gap: 10px; }
  .nav-badge { display: none; }
  .hero { padding: 54px 18px 40px; }
  .hero-grid { grid-template-columns: 1fr; }
  .tutor-board { padding: 16px; }
  .container, .content { padding-left: 18px; padding-right: 18px; }
  .feature-grid { grid-template-columns: 1fr 1fr; }
  .chapter-nav { grid-template-columns: 1fr; }
  .next { text-align: left; }
  .chapter-hero h1 { font-size: 1.72rem; word-break: break-word; }
  .chapter-hero h1 br { display: block; }
}
@media (max-width: 480px) {
  .feature-grid { grid-template-columns: 1fr; }
  .card-list a { padding: 14px; }
  .list-num { width: 30px; height: 30px; }
}