/* ==========================================================================
   LearnAI deep-dive / hub design system — APPLE style.
   Same class names + CSS-variable names as before (so all pages adopt it),
   restyled to SF Pro / white-grey-black bands / teal+purple+blue accents.
   ========================================================================== */

:root{
  /* Apple palette */
  --white:#ffffff; --grey:#f5f5f7; --ink:#1d1d1f; --sub:#6e6e73; --hair:#d2d2d7;
  --blue:#0071e3; --calc:#0a9d94; --llm:#8a4dff;

  /* back-compat names (used by class rules + inline var() styles across pages) */
  --paper:#ffffff; --paper-2:#ffffff;
  --ink-soft:#424245; --muted:#6e6e73;
  --line:#d2d2d7; --line-2:#e8e8ed;
  --blue-ink:#0071e3; --blue-soft:#eef4fd; --blue-border:#cfe0fb;
  --mint-bg:#e6f7f4; --mint-border:#0a9d94; --mint-text:#0a7d75;
  --amber-bg:#fff6e6; --amber-border:#ffb340; --amber-ink:#8a5a00;
  --warn-bg:#ffefef; --warn-border:#ff6b6b; --warn-text:#c0342b;
  --code-bg:#1d1d1f; --code-line:#2a2a2c; --code-text:#f5f5f7; --code-dim:#86868b;
  --tok-kw:#ff7ab2; --tok-fn:#e0c56b; --tok-num:#7fe0a6; --tok-str:#9ad4a0; --tok-com:#7d8590; --out-text:#66d9a0;
  --black:#1d1d1f; --shadow:0 4px 22px rgba(0,0,0,.06); --radius:18px; --maxw:820px;
}

*{box-sizing:border-box}
html{scroll-behavior:smooth}
body{
  margin:0;background:var(--white);color:var(--ink);
  font-family:-apple-system,BlinkMacSystemFont,"SF Pro Display","SF Pro Text","Helvetica Neue",Helvetica,Arial,sans-serif;
  line-height:1.5;letter-spacing:-.01em;-webkit-font-smoothing:antialiased;
}
a{color:var(--blue);text-decoration:none}
a:hover{text-decoration:underline}

/* ---------- top bar (Apple translucent nav) ---------- */
.topbar{position:sticky;top:0;z-index:50;background:rgba(255,255,255,.72);
  backdrop-filter:saturate(180%) blur(20px);-webkit-backdrop-filter:saturate(180%) blur(20px);
  border-bottom:1px solid rgba(0,0,0,.08)}
.topbar .inner{max-width:1024px;margin:0 auto;padding:0 22px;height:52px;display:flex;align-items:center;justify-content:space-between;gap:12px}
.brand{display:flex;align-items:center;gap:10px;font-weight:600;color:var(--ink);letter-spacing:-.01em;font-size:17px}
.brand:hover{text-decoration:none}
.brand .glyph{width:28px;height:28px;border-radius:8px;background:var(--ink);color:#fff;display:grid;place-items:center;font-size:14px;font-weight:700}
.brand small{display:block;font-weight:400;font-size:12px;color:var(--sub);letter-spacing:0}
.topbar .back{font-size:14px;font-weight:400;color:var(--blue);border:0;padding:6px 2px;background:none}
.topbar .back:hover{text-decoration:underline}

/* ---------- layout ---------- */
.wrap{max-width:var(--maxw);margin:0 auto;padding:0 22px 110px}
.hero{padding:64px 0 8px}
.eyebrow{display:inline-block;font-size:15px;font-weight:600;letter-spacing:-.01em;color:var(--blue);margin:0 0 10px;text-transform:none}
h1{font-size:clamp(34px,6vw,56px);line-height:1.05;font-weight:600;letter-spacing:-.022em;margin:0 0 18px;color:var(--ink)}
.lead{font-size:clamp(19px,2.6vw,24px);color:var(--sub);margin:0 0 8px;line-height:1.4;font-weight:400;letter-spacing:-.012em}

h2{font-size:clamp(26px,4vw,38px);line-height:1.1;font-weight:600;letter-spacing:-.02em;margin:64px 0 6px;color:var(--ink);scroll-margin-top:80px}
.section-kicker{font-size:15px;font-weight:600;letter-spacing:-.01em;color:var(--blue);margin:0 0 2px}
h3{font-size:22px;font-weight:600;letter-spacing:-.018em;margin:34px 0 8px;color:var(--ink)}
p{margin:14px 0;color:var(--ink-soft);font-size:17px;line-height:1.55}
strong{color:var(--ink);font-weight:600}
em{font-style:italic}
ul,ol{margin:14px 0;padding-left:22px;color:var(--ink-soft);font-size:17px}
li{margin:8px 0}
li::marker{color:var(--sub)}
hr{border:0;border-top:1px solid var(--line);margin:48px 0}
.section-intro{font-size:19px;color:var(--sub);line-height:1.45}

/* ---------- objective / learning-goal card ---------- */
.objective{background:var(--grey);border:1px solid transparent;border-radius:var(--radius);padding:26px 28px;margin:28px 0}
.objective .label{font-size:14px;font-weight:600;color:var(--blue);margin:0 0 10px;text-transform:none;letter-spacing:-.01em}
.objective h3,.objective h2{margin:0 0 10px}
.objective ul{margin:8px 0 0;padding-left:20px}
.objective li{color:var(--ink)}

/* ---------- callouts ---------- */
.callout{border-radius:16px;padding:18px 20px;margin:24px 0;border:1px solid var(--hair);background:var(--white);box-shadow:var(--shadow)}
.callout .tag{display:flex;align-items:center;gap:9px;font-weight:600;font-size:15px;color:var(--ink);margin:0 0 4px}
.callout .em{font-size:19px;line-height:1}
.callout p{margin:4px 0 0;font-size:16px;color:var(--ink-soft)}
.callout.analogy{background:var(--amber-bg);border-color:#ffe1ab}
.callout.analogy .tag{color:var(--amber-ink)}
.callout.warn{background:var(--warn-bg);border-color:#ffcaca}
.callout.warn .tag{color:var(--warn-text)}
.callout.key{background:var(--mint-bg);border-color:#a9e4db}
.callout.key .tag{color:var(--mint-text)}

/* ---------- code card ---------- */
.code{background:var(--code-bg);border:1px solid var(--code-line);border-radius:16px;padding:0;margin:26px 0;overflow:hidden}
.code .bar{display:flex;align-items:center;gap:7px;padding:12px 18px;border-bottom:1px solid var(--code-line)}
.code .bar .dot{width:11px;height:11px;border-radius:50%;background:#3a3a3c}
.code .bar .name{margin-left:8px;font-size:12px;color:var(--code-dim);font-family:"SF Mono",ui-monospace,Menlo,Consolas,monospace}
.code pre{margin:0;padding:20px;overflow-x:auto;font-family:"SF Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13.5px;line-height:1.7;color:var(--code-text)}
.code .kw{color:var(--tok-kw)}.code .fn{color:var(--tok-fn)}.code .num{color:var(--tok-num)}.code .str{color:var(--tok-str)}.code .com{color:var(--tok-com)}
.inline-code{font-family:"SF Mono",ui-monospace,Menlo,Consolas,monospace;font-size:.9em;background:var(--grey);border:1px solid var(--line);border-radius:6px;padding:.06em .38em;color:var(--ink)}
.output{margin:16px 20px 20px;background:var(--code-line);color:var(--out-text);border-radius:12px;padding:12px 14px;font-family:"SF Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}
.output .lbl{display:block;color:var(--code-dim);font-size:11px;letter-spacing:.02em;text-transform:uppercase;margin-bottom:5px}

/* ---------- comparison tiles ---------- */
.grid2{display:grid;grid-template-columns:1fr 1fr;gap:16px;margin:24px 0}
.tile{background:var(--white);border:1px solid var(--hair);border-radius:16px;padding:20px 22px;box-shadow:var(--shadow)}
.tile h4{margin:2px 0 6px;font-size:17px;font-weight:600;color:var(--ink)}
.tile p{margin:0;font-size:15px;color:var(--ink-soft)}
.tile .pill{display:inline-block;font-size:12px;font-weight:600;padding:3px 11px;border-radius:999px;margin-bottom:10px}
.pill.good{background:var(--mint-bg);color:var(--mint-text)}
.pill.bad{background:var(--warn-bg);color:var(--warn-text)}
.pill.info{background:var(--blue-soft);color:var(--blue-ink)}

/* ---------- checkpoint quiz ---------- */
.checkpoint{background:var(--grey);border:1px solid transparent;border-radius:var(--radius);padding:24px 26px;margin:32px 0}
.checkpoint .label{font-size:14px;font-weight:600;color:var(--blue);margin:0 0 10px}
.checkpoint .q{font-weight:600;font-size:18px;color:var(--ink);margin:0 0 14px;letter-spacing:-.01em}
.answers{display:flex;flex-wrap:wrap;gap:10px}
.answer{font-size:15px;font-weight:500;padding:10px 16px;border-radius:999px;cursor:pointer;background:var(--white);color:var(--ink);border:1px solid var(--hair);transition:.15s}
.answer:hover{border-color:var(--blue);color:var(--blue)}
.answer.correct.reveal{background:var(--mint-bg);color:var(--mint-text);border-color:var(--mint-border)}
.answer.wrong.reveal{opacity:.45}
.explain{display:none;margin:14px 0 0;font-size:16px;color:var(--ink-soft);border-top:1px solid var(--line);padding-top:14px}
.explain.show{display:block}

/* ---------- project / try-it ---------- */
.project{background:var(--grey);border:1px solid transparent;border-radius:var(--radius);padding:26px 28px;margin:36px 0}
.project .label{font-size:14px;font-weight:600;color:var(--calc);margin:0 0 8px}
.project h3{margin:0 0 8px}
.project ol{margin:10px 0 0;padding-left:20px}
.project li{color:var(--ink)}

/* ---------- buttons (Apple pill) ---------- */
.btn{display:inline-flex;align-items:center;gap:8px;font-weight:500;font-size:16px;background:var(--blue);color:#fff;border:none;border-radius:980px;padding:11px 22px;cursor:pointer;transition:.2s}
.btn:hover{text-decoration:none;background:#0077ED;color:#fff}
.btn.ghost{background:transparent;color:var(--blue);border:1px solid var(--line)}
.btn.ghost:hover{background:var(--grey)}

/* ---------- prev / next footer nav ---------- */
.lessonnav{display:grid;grid-template-columns:1fr 1fr;gap:16px;margin:64px 0 0;border-top:1px solid var(--line);padding-top:32px}
.lessonnav a{display:block;padding:18px 20px;border:1px solid var(--hair);border-radius:16px;background:var(--white);transition:.2s}
.lessonnav a:hover{text-decoration:none;border-color:var(--blue);box-shadow:var(--shadow)}
.lessonnav .dir{font-size:13px;font-weight:600;color:var(--sub)}
.lessonnav .ttl{display:block;font-weight:600;color:var(--ink);margin-top:3px;letter-spacing:-.01em}
.lessonnav a.next{text-align:right}
.lessonnav a.disabled{opacity:.4;pointer-events:none}

/* progress dots */
.progress{display:flex;align-items:center;gap:6px;margin:4px 0 30px;flex-wrap:wrap}
.progress .d{width:8px;height:8px;border-radius:50%;background:var(--hair)}
.progress .d.done{background:#aa9fff}
.progress .d.here{background:var(--blue);box-shadow:0 0 0 4px var(--blue-soft)}
.progress .lbl{font-size:14px;color:var(--sub);margin-left:8px;font-weight:400}

/* footer */
.foot{max-width:1024px;margin:0 auto;padding:34px 22px;color:var(--sub);font-size:13px;border-top:1px solid var(--line);background:var(--grey)}
.foot a{color:var(--blue)}

@media (max-width:640px){
  .grid2{grid-template-columns:1fr}
  .lessonnav{grid-template-columns:1fr}
  .lessonnav a.next{text-align:left}
  .hero{padding:44px 0 4px}
}