/* Physics Decoded — site styles. One file, no frameworks. */
:root {
  --bg: #fbf8f3;
  --paper: #ffffff;
  --ink: #15202e;
  --ink-2: #44505f;
  --ink-3: #6b7685;
  --line: #e6e0d6;
  --accent: #e8572a;       /* orange: actions, highlights */
  --accent-ink: #b8401a;
  --blue: #2563c9;         /* velocity */
  --green: #1c8a5a;        /* acceleration */
  --soft-blue: #eaf1fc;
  --soft-orange: #fdefe8;
  --soft-green: #e7f5ee;
  --shadow: 0 1px 2px rgba(20, 30, 45, .06), 0 6px 24px rgba(20, 30, 45, .06);
  --radius: 14px;
  --font: "Segoe UI", system-ui, -apple-system, Roboto, "Helvetica Neue", Arial, sans-serif;
  --mono: ui-monospace, "Cascadia Mono", Consolas, monospace;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root {
    --bg: #0f151d; --paper: #161e29; --ink: #eef2f7; --ink-2: #c3ccd8; --ink-3: #8d99a8; --line: #26313f;
    --accent: #ff7a4a; --accent-ink: #ff9a73; --blue: #6aa3ff; --green: #4fd19a;
    --soft-blue: #17263d; --soft-orange: #2c1d17; --soft-green: #13291f;
    --shadow: 0 1px 2px rgba(0,0,0,.3), 0 8px 28px rgba(0,0,0,.25);
    color-scheme: dark;
  }
}

* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body { margin: 0; background: var(--bg); color: var(--ink); font: 17px/1.65 var(--font); }
img, svg { max-width: 100%; display: block; }
a { color: var(--accent-ink); text-underline-offset: 3px; }
a:hover { color: var(--accent); }
h1, h2, h3 { line-height: 1.2; letter-spacing: -0.01em; margin: 0 0 .5em; }
h1 { font-size: clamp(1.9rem, 4.6vw, 2.9rem); font-weight: 800; }
h2 { font-size: clamp(1.35rem, 2.8vw, 1.7rem); font-weight: 750; margin-top: 2.2em; }
h3 { font-size: 1.1rem; font-weight: 700; }
p { margin: 0 0 1em; }
.wrap { width: min(1080px, 100% - 32px); margin-inline: auto; }
.narrow { width: min(760px, 100% - 32px); margin-inline: auto; }
.skip { position: absolute; left: -999px; }
.skip:focus { left: 12px; top: 12px; background: var(--paper); padding: 8px 12px; z-index: 10; }

/* Header */
.top { position: sticky; top: 0; z-index: 5; background: color-mix(in srgb, var(--bg) 88%, transparent); backdrop-filter: blur(10px); border-bottom: 1px solid var(--line); }
.top .wrap { display: flex; align-items: center; gap: 18px; height: 62px; }
.logo { display: flex; align-items: center; gap: 10px; font-weight: 800; color: var(--ink); text-decoration: none; font-size: 1.05rem; }
.logo-mark { width: 30px; height: 30px; border-radius: 8px; background: var(--accent); position: relative; box-shadow: 4px 4px 0 var(--ink); }
.logo-mark::after { content: ""; position: absolute; inset: 9px; border-radius: 50%; background: var(--paper); }
.nav { display: flex; gap: 18px; margin-left: auto; align-items: center; }
.nav a { color: var(--ink-2); text-decoration: none; font-weight: 600; font-size: .95rem; }
.nav a:hover { color: var(--ink); }
.nav a.btn-yt, .nav a.btn-yt:hover { color: #fff; }
@media (max-width: 640px) { .nav .hide-sm { display: none; } }

/* Buttons */
.btn { display: inline-flex; align-items: center; gap: 8px; padding: 11px 18px; border-radius: 999px; font-weight: 700; text-decoration: none; border: 0; cursor: pointer; font: inherit; font-weight: 700; font-size: .95rem; }
.btn-yt { background: #e62117; color: #fff; }
.btn-yt:hover { background: #c81a11; color: #fff; }
.btn-ghost { background: var(--paper); color: var(--ink); border: 1px solid var(--line); }
.btn-ghost:hover { border-color: var(--ink-3); color: var(--ink); }
.btn svg { width: 18px; height: 18px; }

/* Breadcrumb + meta */
.crumbs { font-size: .9rem; color: var(--ink-3); margin: 28px 0 10px; }
.crumbs a { color: var(--ink-3); text-decoration: none; }
.crumbs a:hover { color: var(--ink); text-decoration: underline; }
.tags { display: flex; flex-wrap: wrap; gap: 8px; margin: 0 0 18px; }
.tag { font-size: .8rem; font-weight: 700; padding: 4px 10px; border-radius: 999px; background: var(--paper); border: 1px solid var(--line); color: var(--ink-2); }

/* Quick answer (the AEO block) */
.answer { background: var(--paper); border: 1px solid var(--line); border-left: 6px solid var(--accent); border-radius: var(--radius); padding: 20px 22px; box-shadow: var(--shadow); margin: 18px 0 26px; }
.answer .label { font-size: .78rem; font-weight: 800; letter-spacing: .08em; text-transform: uppercase; color: var(--accent-ink); margin-bottom: 6px; }
.answer p { font-size: 1.12rem; margin: 0; }
.answer p + p { margin-top: .6em; font-size: 1rem; color: var(--ink-2); }

/* Video facade */
.video { position: relative; aspect-ratio: 16 / 9; border-radius: var(--radius); overflow: hidden; background: #000; box-shadow: var(--shadow); }
.video button { all: unset; position: absolute; inset: 0; cursor: pointer; }
.video img { width: 100%; height: 100%; object-fit: cover; }
.video .play { position: absolute; left: 50%; top: 50%; translate: -50% -50%; width: 76px; height: 54px; border-radius: 14px; background: #e62117; display: grid; place-items: center; box-shadow: 0 6px 20px rgba(0,0,0,.35); transition: transform .15s; }
.video button:hover .play { transform: scale(1.08); }
.video .play::after { content: ""; border-left: 20px solid #fff; border-top: 12px solid transparent; border-bottom: 12px solid transparent; margin-left: 5px; }
.video iframe { position: absolute; inset: 0; width: 100%; height: 100%; border: 0; }
.video-bar { display: flex; flex-wrap: wrap; gap: 10px; align-items: center; justify-content: space-between; margin: 12px 0 0; font-size: .92rem; color: var(--ink-3); }

/* Content blocks */
.card { background: var(--paper); border: 1px solid var(--line); border-radius: var(--radius); padding: 20px 22px; box-shadow: var(--shadow); }
.formula { font-family: var(--mono); font-size: 1.05rem; background: var(--soft-blue); color: var(--ink); padding: 2px 8px; border-radius: 6px; white-space: nowrap; }
.formula-big { display: block; text-align: center; font-family: var(--mono); font-size: 1.35rem; padding: 16px; margin: 12px 0 18px; background: var(--soft-blue); border-radius: 12px; }
.steps { counter-reset: s; list-style: none; padding: 0; margin: 0 0 1em; }
.steps li { counter-increment: s; position: relative; padding-left: 40px; margin-bottom: 10px; }
.steps li::before { content: counter(s); position: absolute; left: 0; top: 1px; width: 26px; height: 26px; border-radius: 50%; background: var(--ink); color: var(--paper); font-size: .85rem; font-weight: 800; display: grid; place-items: center; }
.mistake { background: var(--soft-orange); border-radius: var(--radius); padding: 18px 22px; margin: 18px 0; }
.mistake h3 { color: var(--accent-ink); }
.mistake ul { margin: 0; padding-left: 1.2em; }
.mistake li { margin-bottom: 6px; }
.compare { width: 100%; border-collapse: collapse; margin: 10px 0 20px; font-size: .97rem; }
.compare th, .compare td { text-align: left; padding: 10px 12px; border-bottom: 1px solid var(--line); vertical-align: top; }
.compare th { font-size: .8rem; text-transform: uppercase; letter-spacing: .05em; color: var(--ink-3); }

/* Interactive: ball + graph lab */
.lab { display: grid; grid-template-columns: 1fr 1.35fr; gap: 18px; align-items: stretch; }
@media (max-width: 760px) { .lab { grid-template-columns: 1fr; } }
.lab-panel { background: var(--paper); border: 1px solid var(--line); border-radius: var(--radius); padding: 14px; box-shadow: var(--shadow); }
.lab-panel h3 { font-size: .85rem; text-transform: uppercase; letter-spacing: .06em; color: var(--ink-3); margin: 2px 4px 8px; }
.lab svg text { font-family: var(--font); }
.readout { display: grid; grid-template-columns: repeat(3, 1fr); gap: 8px; margin-top: 10px; }
.readout div { background: var(--bg); border-radius: 10px; padding: 8px 10px; }
.readout b { display: block; font-family: var(--mono); font-size: 1.15rem; }
.readout span { font-size: .75rem; color: var(--ink-3); font-weight: 700; text-transform: uppercase; letter-spacing: .05em; }
.lab-controls { display: flex; gap: 10px; align-items: center; flex-wrap: wrap; margin-top: 12px; }
.lab-controls input[type=range] { flex: 1; min-width: 160px; accent-color: var(--accent); }
.seg { display: inline-flex; background: var(--bg); border: 1px solid var(--line); border-radius: 999px; padding: 3px; }
.seg button { border: 0; background: none; font: inherit; font-weight: 700; font-size: .88rem; padding: 6px 12px; border-radius: 999px; cursor: pointer; color: var(--ink-2); }
.seg button[aria-pressed=true] { background: var(--ink); color: var(--paper); }
.case-note { min-height: 3.2em; margin: 10px 4px 0; color: var(--ink-2); font-size: .95rem; }

/* Quiz */
.quiz { margin: 14px 0; }
.q { background: var(--paper); border: 1px solid var(--line); border-radius: var(--radius); padding: 18px 20px; margin-bottom: 14px; box-shadow: var(--shadow); }
.q p.stem { font-weight: 650; margin-bottom: 12px; }
.opts { display: grid; gap: 8px; }
.opts button { text-align: left; font: inherit; padding: 10px 14px; border-radius: 10px; border: 1.5px solid var(--line); background: var(--bg); color: var(--ink); cursor: pointer; }
.opts button:hover:not(:disabled) { border-color: var(--ink-3); }
.opts button.right { border-color: var(--green); background: var(--soft-green); }
.opts button.wrong { border-color: var(--accent); background: var(--soft-orange); }
.opts button:disabled { cursor: default; }
.why { margin-top: 12px; font-size: .95rem; color: var(--ink-2); display: none; }
.why.show { display: block; }

/* FAQ */
details { background: var(--paper); border: 1px solid var(--line); border-radius: 12px; padding: 14px 18px; margin-bottom: 10px; }
details summary { cursor: pointer; font-weight: 700; list-style: none; display: flex; justify-content: space-between; gap: 12px; }
details summary::-webkit-details-marker { display: none; }
details summary::after { content: "+"; font-weight: 800; color: var(--accent); }
details[open] summary::after { content: "−"; }
details p { margin: 10px 0 0; color: var(--ink-2); }

/* Subscribe band */
.sub { margin: 44px 0; background: var(--ink); color: var(--paper); border-radius: 20px; padding: 28px; display: flex; gap: 22px; align-items: center; flex-wrap: wrap; }
.sub h2 { margin: 0 0 6px; color: var(--paper); }
.sub p { margin: 0; color: color-mix(in srgb, var(--paper) 75%, transparent); }
.sub .grow { flex: 1; min-width: 240px; }

/* Next / prev */
.pager { display: grid; grid-template-columns: 1fr 1fr; gap: 14px; margin: 26px 0; }
.pager a { display: block; padding: 16px 18px; border: 1px solid var(--line); border-radius: var(--radius); text-decoration: none; color: var(--ink); background: var(--paper); }
.pager a:hover { border-color: var(--ink-3); }
.pager small { display: block; color: var(--ink-3); font-weight: 700; font-size: .8rem; text-transform: uppercase; letter-spacing: .05em; }
.pager .next { text-align: right; }
@media (max-width: 560px) { .pager { grid-template-columns: 1fr; } .pager .next { text-align: left; } }

/* Transcript */
.transcript { max-height: 340px; overflow: auto; font-size: .95rem; color: var(--ink-2); padding-right: 6px; }

/* Home */
.hero { padding: 64px 0 36px; display: grid; grid-template-columns: 1.15fr 1fr; gap: 40px; align-items: center; }
@media (max-width: 860px) { .hero { grid-template-columns: 1fr; padding-top: 36px; } }
.hero .kicker { font-weight: 800; color: var(--accent-ink); letter-spacing: .06em; text-transform: uppercase; font-size: .82rem; }
.hero p.lead { font-size: 1.2rem; color: var(--ink-2); max-width: 34em; }
.hero-actions { display: flex; gap: 12px; flex-wrap: wrap; margin-top: 22px; }
.search { display: flex; gap: 8px; background: var(--paper); border: 1.5px solid var(--line); border-radius: 999px; padding: 6px 6px 6px 18px; margin-top: 22px; box-shadow: var(--shadow); max-width: 520px; }
.search input { flex: 1; border: 0; outline: 0; background: none; font: inherit; color: var(--ink); min-width: 0; }
.search:focus-within { border-color: var(--accent); }
.results { max-width: 520px; margin-top: 8px; }
.results a { display: block; padding: 8px 14px; border-radius: 10px; text-decoration: none; color: var(--ink); }
.results a:hover { background: var(--paper); }
.qa-stack { display: grid; gap: 12px; }
.qa-chip { background: var(--paper); border: 1px solid var(--line); border-radius: 14px; padding: 14px 16px; box-shadow: var(--shadow); text-decoration: none; color: var(--ink); display: block; }
.qa-chip:nth-child(2) { margin-left: 28px; }
.qa-chip:nth-child(3) { margin-left: 8px; }
.qa-chip b { display: block; }
.qa-chip span { color: var(--ink-3); font-size: .9rem; }
.qa-chip:hover { border-color: var(--accent); }
.section-head { display: flex; justify-content: space-between; align-items: end; gap: 12px; margin: 40px 0 14px; }
.section-head h2 { margin: 0; }
.grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(250px, 1fr)); gap: 16px; }
.lesson-card { background: var(--paper); border: 1px solid var(--line); border-radius: var(--radius); overflow: hidden; text-decoration: none; color: var(--ink); box-shadow: var(--shadow); display: flex; flex-direction: column; }
.lesson-card:hover { border-color: var(--ink-3); }
.lesson-card img { aspect-ratio: 16/9; object-fit: cover; width: 100%; background: var(--line); }
.lesson-card .body { padding: 14px 16px 16px; }
.lesson-card small { color: var(--ink-3); font-weight: 700; font-size: .78rem; text-transform: uppercase; letter-spacing: .05em; }
.lesson-card b { display: block; margin: 4px 0 6px; line-height: 1.3; }
.lesson-card p { margin: 0; font-size: .92rem; color: var(--ink-2); }
.lesson-card.soon { opacity: .6; pointer-events: none; }
.chapters { display: grid; grid-template-columns: repeat(auto-fill, minmax(220px, 1fr)); gap: 10px; }
.chapters a { padding: 12px 14px; border: 1px solid var(--line); border-radius: 12px; background: var(--paper); text-decoration: none; color: var(--ink); font-weight: 650; }
.chapters a span { display: block; color: var(--ink-3); font-weight: 500; font-size: .85rem; }
.how { display: grid; grid-template-columns: repeat(auto-fit, minmax(220px, 1fr)); gap: 16px; }
.how .card b { display: block; margin-bottom: 4px; }

footer { border-top: 1px solid var(--line); margin-top: 60px; padding: 28px 0 40px; color: var(--ink-3); font-size: .9rem; }
footer .wrap { display: flex; justify-content: space-between; gap: 16px; flex-wrap: wrap; }
footer a { color: var(--ink-3); }