/* /learn — pure B&W course system, VV-inspired */
:root{
  --bg:#ffffff;
  --ink:#111111;
  --muted:#6b7280;
  --line:#e5e7eb;
  --line-strong:#111111;
  --hover:#f5f5f5;
}
*{box-sizing:border-box;margin:0;padding:0}
html{-webkit-font-smoothing:antialiased;text-size-adjust:100%}
body{
  font-family:-apple-system,BlinkMacSystemFont,"Inter","SF Pro Display","Segoe UI",system-ui,sans-serif;
  background:var(--bg);
  color:var(--ink);
  line-height:1.6;
  font-size:16px;
  letter-spacing:-0.003em;
}
a{color:inherit;text-decoration:none}

/* NAV */
.lrn-nav{
  position:sticky;top:0;z-index:50;
  background:rgba(255,255,255,.92);
  backdrop-filter:blur(12px);
  -webkit-backdrop-filter:blur(12px);
  border-bottom:1px solid var(--line);
}
.lrn-nav-inner{
  max-width:1200px;margin:0 auto;padding:16px 32px;
  display:flex;align-items:center;justify-content:space-between;gap:24px;
}
.lrn-brand{font-weight:800;font-size:1.05rem;letter-spacing:-0.02em}
.lrn-brand b{font-weight:800}
.lrn-nav-links{display:flex;gap:28px;align-items:center;font-size:.95rem;color:var(--muted)}
.lrn-nav-links a:hover{color:var(--ink)}
.lrn-nav-links a.active{color:var(--ink);font-weight:600}
@media(max-width:700px){
  .lrn-nav-inner{padding:14px 20px}
  .lrn-nav-links{gap:16px;font-size:.85rem}
}

/* CATALOG */
.lrn-wrap{max-width:1200px;margin:0 auto;padding:80px 32px 120px}
.lrn-page-title{font-size:clamp(2.2rem,4.5vw,3.6rem);font-weight:800;letter-spacing:-0.04em;line-height:1.05;margin-bottom:16px}
.lrn-page-sub{color:var(--muted);font-size:1.1rem;max-width:640px;margin-bottom:64px;line-height:1.55}

.lrn-grid{
  display:grid;
  grid-template-columns:repeat(auto-fill,minmax(320px,1fr));
  gap:32px;
}
.lrn-card{
  border:1px solid var(--line);
  border-radius:16px;
  overflow:hidden;
  background:var(--bg);
  display:flex;flex-direction:column;
  transition:transform .18s ease,border-color .18s ease;
}
.lrn-card:hover{transform:translateY(-2px);border-color:var(--line-strong)}
.lrn-card-img{
  aspect-ratio:16/10;background:#000;display:block;
}
.lrn-card-img svg{width:100%;height:100%;display:block}
.lrn-card-body{padding:24px}
.lrn-card-tag{font-size:.7rem;letter-spacing:2.5px;text-transform:uppercase;color:var(--muted);margin-bottom:12px;font-weight:600}
.lrn-card-title{font-size:1.35rem;font-weight:800;letter-spacing:-0.02em;margin-bottom:8px;line-height:1.2}
.lrn-card-desc{color:var(--muted);font-size:.95rem;line-height:1.5;margin-bottom:20px}
.lrn-card-meta{display:flex;gap:16px;font-size:.85rem;color:var(--muted);align-items:center}
.lrn-card-meta b{color:var(--ink);font-weight:700}
.lrn-card.soon{opacity:.55;pointer-events:none}
.lrn-card.soon .lrn-card-img{background:#f3f4f6}
.lrn-card.soon .lrn-card-title{color:var(--muted)}

/* COURSE HOME */
.course-hero{padding:60px 0 48px;max-width:760px}
.course-back{font-size:.95rem;color:var(--muted);display:inline-block;margin-bottom:32px}
.course-back:hover{color:var(--ink)}
.course-title{font-size:clamp(2.2rem,5vw,3.4rem);font-weight:800;letter-spacing:-0.04em;line-height:1.05;margin-bottom:20px}
.course-desc{color:var(--muted);font-size:1.15rem;line-height:1.55;margin-bottom:28px;max-width:600px}
.course-stats{display:flex;gap:18px;align-items:center;font-size:.95rem;color:var(--muted);flex-wrap:wrap;margin-bottom:28px}
.course-stats b{color:var(--ink);font-weight:700;margin-right:4px}
.course-stats .sep{opacity:.4}
.progress-row{display:flex;align-items:center;gap:16px;margin-bottom:28px;max-width:600px}
.progress-bar{flex:1;height:3px;background:var(--line);border-radius:2px;overflow:hidden}
.progress-bar .fill{height:100%;background:var(--ink);width:0%;transition:width .3s ease}
.progress-text{font-size:.85rem;color:var(--muted);font-variant-numeric:tabular-nums;font-weight:600}
.btn-start{
  display:inline-block;background:var(--ink);color:#fff;
  padding:14px 28px;border-radius:999px;
  font-weight:600;font-size:1rem;letter-spacing:-0.01em;
  transition:opacity .18s ease;
}
.btn-start:hover{opacity:.85}

.preview-tiles{
  display:grid;grid-template-columns:repeat(3,1fr);gap:20px;
  margin:56px 0;max-width:760px;
}
.preview-tiles .tile{aspect-ratio:1;background:#000;border-radius:10px;overflow:hidden}
.preview-tiles .tile svg{width:100%;height:100%;display:block}
@media(max-width:640px){.preview-tiles{grid-template-columns:1fr 1fr}}

.modules{max-width:760px;margin-top:40px}
.module-block{margin-bottom:44px}
.module-label{font-size:.7rem;letter-spacing:2.5px;text-transform:uppercase;color:var(--muted);font-weight:600;margin-bottom:20px}
.lesson-row{
  display:flex;align-items:center;gap:14px;
  padding:10px 0;font-size:1.02rem;
  color:var(--ink);
}
.lesson-row .circle{
  width:18px;height:18px;border:1.5px solid var(--line);border-radius:50%;
  flex-shrink:0;display:flex;align-items:center;justify-content:center;
  transition:border-color .18s,background .18s;
}
.lesson-row.done .circle{background:var(--ink);border-color:var(--ink)}
.lesson-row.done .circle::after{
  content:'';width:8px;height:4px;
  border-left:1.5px solid #fff;border-bottom:1.5px solid #fff;
  transform:rotate(-45deg) translate(1px,-1px);
}
.lesson-row:hover{color:#000}
.lesson-row:hover .circle{border-color:var(--ink)}
.lesson-row.done:hover .circle{background:#000}

/* LESSON PAGE */
.lesson-layout{
  display:grid;
  grid-template-columns:260px 1fr;
  gap:64px;
  max-width:1100px;margin:0 auto;padding:40px 32px 120px;
  align-items:start;
}
.sidebar{
  position:sticky;top:84px;
  max-height:calc(100vh - 100px);
  overflow-y:auto;
  font-size:.92rem;
  padding-right:8px;
  scrollbar-width:thin;
}
.sidebar::-webkit-scrollbar{width:6px}
.sidebar::-webkit-scrollbar-thumb{background:var(--line);border-radius:3px}
.sidebar-back{font-size:.85rem;color:var(--muted);display:inline-block;margin-bottom:20px}
.sidebar-back:hover{color:var(--ink)}
.sidebar .module-label{font-size:.65rem;letter-spacing:2.5px;margin-top:20px;margin-bottom:10px}
.sidebar .module-label:first-of-type{margin-top:0}
.sidebar .lesson-row{padding:6px 0;font-size:.92rem;color:var(--muted);gap:10px}
.sidebar .lesson-row .circle{width:14px;height:14px;border-width:1.2px}
.sidebar .lesson-row.done .circle::after{width:6px;height:3px;transform:rotate(-45deg) translate(0,-1px)}
.sidebar .lesson-row.current{color:var(--ink);font-weight:600}
.sidebar .lesson-row.current .circle{border-color:var(--ink)}
.sidebar .lesson-row.done{color:var(--ink)}
.sidebar .lesson-row:hover{color:var(--ink)}

.lesson-main{max-width:660px}
.lesson-eyebrow{font-size:.75rem;letter-spacing:2.5px;text-transform:uppercase;color:var(--muted);font-weight:600;margin-bottom:10px}
.lesson-title{font-size:clamp(1.8rem,3.8vw,2.7rem);font-weight:800;letter-spacing:-0.03em;line-height:1.1;margin-bottom:36px}

.lesson-main p{font-size:1.06rem;line-height:1.75;margin-bottom:20px;color:#1f2937}
.lesson-main h2{font-size:1.5rem;font-weight:800;letter-spacing:-0.02em;margin-top:44px;margin-bottom:16px;line-height:1.25}
.lesson-main h3{font-size:1.15rem;font-weight:700;letter-spacing:-0.01em;margin-top:28px;margin-bottom:10px}
.lesson-main ul, .lesson-main ol{margin:0 0 20px 24px;color:#1f2937}
.lesson-main li{margin-bottom:8px;line-height:1.65}
.lesson-main strong{font-weight:700;color:var(--ink)}
.lesson-main blockquote{
  border-left:2px solid var(--ink);
  padding:6px 0 6px 20px;
  margin:24px 0;
  font-size:1.12rem;
  font-weight:500;
  color:var(--ink);
  letter-spacing:-0.01em;
}

.lesson-main table{
  width:100%;border-collapse:collapse;margin:24px 0;font-size:.95rem;
}
.lesson-main th,.lesson-main td{
  padding:10px 12px;text-align:left;border-bottom:1px solid var(--line);
}
.lesson-main th{font-weight:700;font-size:.8rem;text-transform:uppercase;letter-spacing:1.2px;color:var(--muted);border-bottom:1.5px solid var(--ink)}
.lesson-main td{color:#1f2937}

.lesson-main .svg-frame{
  background:#000;border-radius:12px;margin:28px 0;overflow:hidden;
}
.lesson-main .svg-frame svg{width:100%;height:auto;display:block}
.lesson-main .svg-caption{
  font-size:.85rem;color:var(--muted);margin-top:-12px;margin-bottom:28px;
  text-align:center;font-style:italic;
}

.lesson-main .callout{
  border:1px solid var(--line-strong);
  border-radius:10px;
  padding:20px 24px;
  margin:28px 0;
  background:var(--bg);
}
.lesson-main .callout b{display:block;font-size:.7rem;letter-spacing:2.5px;text-transform:uppercase;color:var(--muted);font-weight:700;margin-bottom:10px}
.lesson-main .callout p{margin-bottom:0;font-size:1.02rem}

.lesson-nav{
  display:flex;justify-content:space-between;gap:24px;
  margin-top:64px;padding-top:24px;border-top:1px solid var(--line);
  font-size:.95rem;
}
.lesson-nav a{color:var(--muted);display:inline-flex;align-items:center;gap:6px}
.lesson-nav a:hover{color:var(--ink)}
.lesson-nav a.next{margin-left:auto}
.lesson-nav a b{font-weight:600}

.mark-complete{
  margin-top:40px;padding:14px 26px;border-radius:999px;
  background:var(--ink);color:#fff;border:none;cursor:pointer;
  font:inherit;font-weight:600;font-size:.95rem;
  transition:opacity .18s;
}
.mark-complete:hover{opacity:.85}
.mark-complete.done{background:var(--bg);color:var(--ink);border:1px solid var(--line-strong)}

@media(max-width:900px){
  .lesson-layout{grid-template-columns:1fr;gap:32px;padding:24px 20px 80px}
  .sidebar{position:static;max-height:none;overflow:visible;padding-right:0;border-bottom:1px solid var(--line);padding-bottom:20px}
}
@media(max-width:640px){
  .lrn-wrap{padding:48px 20px 80px}
  .course-hero{padding:32px 0 32px}
}

/* LANGUAGE TOGGLE */
.lang-toggle{display:inline-flex;align-items:center;border:1px solid var(--line);border-radius:999px;padding:3px;font-size:.78rem;font-weight:600;letter-spacing:.02em;background:var(--bg);margin-left:12px}
.lang-toggle button{border:none;background:transparent;padding:5px 12px;border-radius:999px;cursor:pointer;color:var(--muted);font:inherit;transition:background .15s,color .15s}
.lang-toggle button.active{background:var(--ink);color:#fff}
.lang-toggle button:hover:not(.active){color:var(--ink)}

/* Language-conditional visibility */
html[lang="zh"] .lang-en{display:none !important}
html[lang="en"] .lang-zh{display:none !important}
/* inline elements should re-appear as inline when shown */
.lang-en, .lang-zh{display:inherit}

/* Chinese font tuning */
html[lang="zh"] body{
  font-family:-apple-system,BlinkMacSystemFont,"PingFang SC","Hiragino Sans GB","Microsoft YaHei","Source Han Sans SC","Noto Sans SC","Inter",system-ui,sans-serif;
  letter-spacing:0;
}
html[lang="zh"] .lesson-title,
html[lang="zh"] .course-title,
html[lang="zh"] .lrn-page-title{letter-spacing:-0.01em}

/* FOOTER */
.lrn-footer{border-top:1px solid var(--line);padding:40px 32px;text-align:center;color:var(--muted);font-size:.85rem}
.lrn-footer a{color:var(--muted)}
.lrn-footer a:hover{color:var(--ink)}