/* Layout: a pocket lab notebook on graph paper. One column with a bottom tab bar on phones;
   a strand rail plus a reading column on wide screens. Sodium-lamp amber marks what you act on. */
:root {
  --paper: #f2f5f9;
  --grid: #dde4ee;
  --card: #ffffff;
  --ink: #131f37;
  --muted: #56647d;
  --line: #c9d3e1;
  --sodium: #b86e00;      /* sodium D line, 589 nm: the accent */
  --sodium-soft: #fbe9c8;
  --mercury: #4146c9;     /* mercury 436 nm: secondary, used in diagrams */
  --mercury-soft: #e2e3fb;
  --good: #1d7f53;
  --good-soft: #d8f0e4;
  --bad: #b8382f;
  --bad-soft: #f8dcd9;
  --hl: #7a3fb0;
  --f-display: "Bricolage Grotesque", "Avenir Next", "Segoe UI", system-ui, sans-serif;
  --f-body: "Atkinson Hyperlegible", "Segoe UI", system-ui, sans-serif;
  --f-math: "STIX Two Text", "Cambria Math", "Times New Roman", serif;
  --f-mono: "JetBrains Mono", ui-monospace, "SF Mono", Menlo, monospace;
  --r: 10px;
  --shadow: 0 1px 0 rgba(19,31,55,.06), 0 6px 20px -12px rgba(19,31,55,.25);
  --topbar-h: 58px;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --paper: #0d131f; --grid: #182234; --card: #141c2b; --ink: #e5eaf3; --muted: #95a2ba; --line: #2a3650;
    --sodium: #f3b240; --sodium-soft: #3a2c12; --mercury: #9599ff; --mercury-soft: #232655;
    --good: #4cc98f; --good-soft: #12382a; --bad: #ff7d72; --bad-soft: #43201d; --hl: #c39cf0;
    --shadow: 0 1px 0 rgba(0,0,0,.3), 0 8px 24px -14px rgba(0,0,0,.8);
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --paper: #0d131f; --grid: #182234; --card: #141c2b; --ink: #e5eaf3; --muted: #95a2ba; --line: #2a3650;
  --sodium: #f3b240; --sodium-soft: #3a2c12; --mercury: #9599ff; --mercury-soft: #232655;
  --good: #4cc98f; --good-soft: #12382a; --bad: #ff7d72; --bad-soft: #43201d; --hl: #c39cf0;
  --shadow: 0 1px 0 rgba(0,0,0,.3), 0 8px 24px -14px rgba(0,0,0,.8);
  color-scheme: dark;
}

* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  background-color: var(--paper);
  background-image:
    linear-gradient(var(--grid) 1px, transparent 1px),
    linear-gradient(90deg, var(--grid) 1px, transparent 1px);
  background-size: 24px 24px;
  background-attachment: fixed;
  color: var(--ink);
  font: 16px/1.6 var(--f-body);
  margin: 0;
}
body::before { content: ""; position: fixed; inset: 0 0 auto; height: env(safe-area-inset-top, 0px); z-index: 10; background: var(--card); }
a { color: var(--sodium); }
button { font: inherit; color: inherit; }
:focus-visible { outline: 2.5px solid var(--sodium); outline-offset: 2px; border-radius: 4px; }
h1, h2, h3, h4 { font-family: var(--f-display); line-height: 1.15; text-wrap: balance; margin: 0; letter-spacing: -0.01em; }
p { margin: 0; }
.m, .eq { font-family: var(--f-math); }
.m i, .eq i { font-style: italic; }
sub, sup { font-size: .72em; line-height: 0; }
[hidden] { display: none !important; }

/* top bar */
.topbar {
  position: sticky; top: env(safe-area-inset-top, 0px); z-index: 20;
  background: color-mix(in srgb, var(--card) 92%, transparent);
  backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--line);
}
.topbar-in {
  max-width: 1180px; margin: 0 auto;
  padding-block: 10px;
  padding-left: max(16px, env(safe-area-inset-left, 0px));
  padding-right: max(16px, env(safe-area-inset-right, 0px));
  display: flex; align-items: center; gap: 14px;
}
.brand { display: flex; align-items: baseline; gap: 8px; text-decoration: none; color: var(--ink); }
.brand b { font: 800 22px/1 var(--f-display); letter-spacing: -0.03em; }
.brand b span { color: var(--sodium); }
.brand small { font: 600 11px/1 var(--f-mono); color: var(--muted); letter-spacing: .06em; text-transform: uppercase; }
.topnav { display: flex; gap: 4px; margin-left: auto; }
.topnav a, .iconbtn {
  text-decoration: none; color: var(--muted); font-weight: 700; font-size: 14px;
  padding: 7px 12px; border-radius: 999px; border: 1px solid transparent; background: none; cursor: pointer;
}
.topnav a:hover, .iconbtn:hover { color: var(--ink); background: var(--paper); }
.topnav a[aria-current="page"] { color: var(--ink); background: var(--sodium-soft); }
.iconbtn { padding: 7px 10px; min-width: 38px; min-height: 36px; }
.topbar-actions { display: flex; gap: 6px; align-items: center; }
.acct { display: inline-flex; align-items: center; gap: 6px; font: 600 12px/1 var(--f-mono); color: var(--muted); padding: 7px 10px; border-radius: 999px; border: 1px solid var(--line); background: var(--card); cursor: pointer; max-width: 180px; }
.acct span { overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
.acct .lv { color: var(--hl); flex: none; }
@media (max-width: 760px) { .topnav { display: none; } .topbar-actions { margin-left: auto; } .brand small { display: none; } .acct { max-width: 120px; } }

/* bottom tab bar on phones */
.tabbar { display: none; }
@media (max-width: 760px) {
  .tabbar {
    display: grid; grid-template-columns: repeat(5, 1fr);
    position: fixed; left: 0; right: 0; bottom: 0; z-index: 30;
    background: var(--card); border-top: 1px solid var(--line);
    padding-bottom: env(safe-area-inset-bottom, 0px);
    padding-left: env(safe-area-inset-left, 0px); padding-right: env(safe-area-inset-right, 0px);
  }
  .tabbar a { display: flex; flex-direction: column; align-items: center; gap: 2px; padding: 8px 2px 9px; text-decoration: none; color: var(--muted); font-size: 11px; font-weight: 700; }
  .tabbar a svg { width: 22px; height: 22px; }
  .tabbar a[aria-current="page"] { color: var(--sodium); }
  body { padding-bottom: calc(64px + env(safe-area-inset-bottom, 0px)); }
}

.wrap {
  max-width: 1180px; margin: 0 auto;
  padding-block: 22px 60px;
  padding-left: max(16px, env(safe-area-inset-left, 0px));
  padding-right: max(16px, env(safe-area-inset-right, 0px));
}
.layout { display: grid; grid-template-columns: 250px minmax(0, 1fr); gap: 28px; align-items: start; }
@media (max-width: 960px) { .layout { grid-template-columns: minmax(0,1fr); } .rail { display: none; } }
.rail { position: sticky; top: calc(env(safe-area-inset-top, 0px) + var(--topbar-h) + 14px); max-height: calc(100vh - var(--topbar-h) - 40px); overflow: auto; padding-right: 4px; }
.rail h4 { font: 700 11px/1.3 var(--f-mono); letter-spacing: .08em; text-transform: uppercase; color: var(--muted); margin: 16px 0 6px; }
.rail h4:first-child { margin-top: 0; }
.rail a { display: flex; align-items: center; gap: 8px; text-decoration: none; color: var(--ink); font-size: 14px; padding: 5px 8px; border-radius: 7px; }
.rail a:hover { background: var(--card); }
.rail a[aria-current="page"] { background: var(--card); box-shadow: var(--shadow); font-weight: 700; }
.rail .secs { display: flex; flex-direction: column; margin: 2px 0 6px 14px; border-left: 2px solid var(--line); }
.rail .secs a { font-size: 13px; padding: 3px 8px; color: var(--muted); border-radius: 0 7px 7px 0; }
.rail .secs a[aria-current="true"] { color: var(--ink); font-weight: 700; background: none; box-shadow: none; }
.dot { width: 9px; height: 9px; border-radius: 50%; flex: none; border: 1.5px solid var(--line); }
.dot.p1 { background: var(--sodium-soft); border-color: var(--sodium); }
.dot.p2 { background: var(--sodium); border-color: var(--sodium); }
.dot.p3 { background: var(--good); border-color: var(--good); }

main { min-width: 0; }
.panel { background: var(--card); border: 1px solid var(--line); border-radius: var(--r); box-shadow: var(--shadow); }
.eyebrow { font: 600 12px/1.3 var(--f-mono); letter-spacing: .08em; text-transform: uppercase; color: var(--muted); }
.chip { display: inline-flex; align-items: center; gap: 5px; font: 700 11px/1 var(--f-mono); letter-spacing: .05em; padding: 4px 7px; border-radius: 5px; border: 1px solid var(--line); color: var(--muted); background: var(--card); white-space: nowrap; }
.chip.hl { color: var(--hl); border-color: color-mix(in srgb, var(--hl) 45%, transparent); }
.chip.acc { color: var(--sodium); border-color: color-mix(in srgb, var(--sodium) 45%, transparent); background: var(--sodium-soft); }
.chip.good { color: var(--good); border-color: color-mix(in srgb, var(--good) 45%, transparent); background: var(--good-soft); }
.btn {
  display: inline-flex; align-items: center; justify-content: center; gap: 8px;
  font-weight: 700; font-size: 15px; padding: 10px 16px; border-radius: 9px; cursor: pointer;
  border: 1.5px solid var(--ink); background: var(--ink); color: var(--card); text-decoration: none; min-height: 40px;
}
.btn:hover { filter: brightness(1.12); }
.btn.acc { background: var(--sodium); border-color: var(--sodium); color: #1b1203; }
.btn.ghost { background: transparent; color: var(--ink); border-color: var(--line); }
.btn.ghost:hover { border-color: var(--ink); filter: none; }
.btn.small { font-size: 13px; padding: 6px 11px; border-radius: 7px; min-height: 32px; }
.btn:disabled { opacity: .45; cursor: default; filter: none; }
.row { display: flex; flex-wrap: wrap; gap: 10px; align-items: center; }
.between { justify-content: space-between; }
.hint { font-size: 13px; color: var(--muted); }
.loading { display: flex; align-items: center; gap: 10px; color: var(--muted); padding: 24px; justify-content: center; }
.loading::before { content: ""; width: 18px; height: 18px; border-radius: 50%; border: 2.5px solid var(--line); border-top-color: var(--sodium); animation: spin .8s linear infinite; }
@keyframes spin { to { transform: rotate(360deg); } }
.notice { background: var(--sodium-soft); border-radius: 9px; padding: 12px 14px; display: flex; gap: 10px; align-items: center; flex-wrap: wrap; margin-bottom: 14px; }
.notice.warn { background: var(--bad-soft); }

/* home */
.hero { display: grid; grid-template-columns: minmax(0, 1.25fr) minmax(0, 1fr); gap: 22px; align-items: stretch; margin-bottom: 30px; }
@media (max-width: 860px) { .hero { grid-template-columns: minmax(0,1fr); } }
.hero-copy { padding: 6px 0; display: flex; flex-direction: column; gap: 14px; justify-content: center; }
.hero h1 { font-size: clamp(34px, 6vw, 60px); font-weight: 800; letter-spacing: -0.035em; line-height: .98; }
.hero h1 em { font-style: normal; color: var(--sodium); }
.hero p.lede { font-size: 18px; color: var(--muted); max-width: 52ch; }
.today { padding: 18px; display: flex; flex-direction: column; gap: 14px; }
.today-stats { display: grid; grid-template-columns: repeat(3, 1fr); gap: 10px; }
.stat { border: 1px solid var(--line); border-radius: 8px; padding: 10px; }
.stat b { display: block; font: 800 28px/1 var(--f-display); font-variant-numeric: tabular-nums; }
.stat span { font-size: 12px; color: var(--muted); }
.spectrum { height: 26px; border-radius: 6px; position: relative; overflow: hidden; background: #05070c; }
.spectrum i { position: absolute; top: 0; bottom: 0; width: 2px; }
.spectrum-cap { display: flex; justify-content: space-between; font: 11px/1.2 var(--f-mono); color: var(--muted); }

.strand { margin-top: 34px; }
.strand-head { display: grid; grid-template-columns: auto minmax(0,1fr); gap: 4px 16px; align-items: baseline; margin-bottom: 14px; }
.strand-num { grid-row: span 2; font: 800 54px/0.9 var(--f-display); color: var(--sodium); letter-spacing: -0.04em; font-variant-numeric: tabular-nums; }
.strand-head h2 { font-size: 26px; }
.strand-head p { color: var(--muted); max-width: 70ch; }
.topic-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(min(100%, 260px), 1fr)); gap: 12px; }
.tcard { display: flex; flex-direction: column; gap: 8px; padding: 14px 14px 12px; text-decoration: none; color: var(--ink); transition: transform .15s ease, border-color .15s ease; }
.tcard:hover { transform: translateY(-2px); border-color: var(--sodium); }
.tcard h3 { font-size: 18px; }
.tcard p { color: var(--muted); font-size: 14px; line-height: 1.45; flex: 1; }
.meter { height: 6px; border-radius: 3px; background: var(--paper); border: 1px solid var(--line); overflow: hidden; }
.meter i { display: block; height: 100%; background: var(--sodium); border-radius: 3px; }
.meter.good i { background: var(--good); }
.tcard-foot { display: flex; justify-content: space-between; align-items: center; gap: 8px; font: 12px/1 var(--f-mono); color: var(--muted); }
.search { width: 100%; max-width: 360px; font: inherit; padding: 9px 12px; border-radius: 9px; border: 1.5px solid var(--line); background: var(--card); color: var(--ink); }
.search:focus { border-color: var(--sodium); outline: none; }

/* topic page */
.thead { display: flex; flex-direction: column; gap: 10px; margin-bottom: 14px; }
.thead h1 { font-size: clamp(28px, 5vw, 46px); font-weight: 800; letter-spacing: -0.03em; }
.thead p.lede { font-size: 17px; color: var(--muted); max-width: 64ch; }
.seg { position: sticky; top: calc(env(safe-area-inset-top, 0px) + var(--topbar-h)); z-index: 5; background: var(--paper); padding: 6px 0 10px; margin: 0 0 14px; }
.seg-in { display: flex; flex-wrap: wrap; gap: 6px; }
.seg button { background: var(--card); border: 1.5px solid var(--line); border-radius: 999px; padding: 7px 13px; font-weight: 700; font-size: 14px; color: var(--muted); cursor: pointer; min-height: 36px; display: inline-flex; gap: 6px; align-items: center; }
.seg button[aria-selected="true"] { color: var(--ink); border-color: var(--sodium); background: var(--sodium-soft); }
.seg button .n { font: 600 11px var(--f-mono); color: var(--muted); }
.seg button.done .n { color: var(--good); }

.secnav { display: flex; gap: 6px; overflow-x: auto; padding-bottom: 6px; margin-bottom: 14px; scrollbar-width: thin; }
.secnav a { flex: none; font-size: 13px; padding: 5px 10px; border-radius: 999px; border: 1px solid var(--line); background: var(--card); color: var(--muted); text-decoration: none; }
.secnav a:hover { color: var(--ink); }
@media (min-width: 961px) { .secnav { display: none; } }

.sec { padding: 22px; margin-bottom: 16px; scroll-margin-top: calc(env(safe-area-inset-top, 0px) + var(--topbar-h) + 70px); }
.sec > h2 { font-size: 24px; margin-bottom: 4px; display: flex; gap: 10px; align-items: baseline; flex-wrap: wrap; }
.sec-body { display: flex; flex-direction: column; gap: 12px; max-width: 70ch; }
.sec-body p, .sec-body li { font-size: 16.5px; }
.sec-body ul, .sec-body ol { margin: 0; padding-left: 22px; display: flex; flex-direction: column; gap: 6px; }
.sec-body strong { color: var(--ink); }
.sec-body .eq, .eqblock {
  font-size: 22px; padding: 12px 16px; background: var(--paper); border-radius: 8px; border-left: 3px solid var(--sodium);
  overflow-x: auto; white-space: nowrap;
}
.key { background: var(--sodium-soft); border-radius: 8px; padding: 12px 14px; }
.key b:first-child { font: 700 11px/1 var(--f-mono); letter-spacing: .08em; text-transform: uppercase; color: var(--sodium); display: block; margin-bottom: 6px; }
.worked { border: 1.5px dashed var(--line); border-radius: 8px; padding: 12px 14px; display: flex; flex-direction: column; gap: 8px; }
.worked > b:first-child { font: 700 11px/1 var(--f-mono); letter-spacing: .08em; text-transform: uppercase; color: var(--mercury); }
.worked ol { padding-left: 20px; }
.analogy { border-left: 3px solid var(--mercury); padding: 4px 0 4px 14px; font-style: italic; color: var(--ink); }
table.data { border-collapse: collapse; font-size: 14.5px; min-width: 100%; }
table.data th, table.data td { border-bottom: 1px solid var(--line); padding: 7px 10px; text-align: left; vertical-align: top; }
table.data th { font: 700 11px/1.3 var(--f-mono); letter-spacing: .06em; text-transform: uppercase; color: var(--muted); }
.tablewrap { overflow-x: auto; position: relative; }
.tablewrap.scrolls { background: linear-gradient(90deg, var(--card) 70%, transparent), linear-gradient(270deg, var(--card) 70%, transparent) 100% 0, radial-gradient(farthest-side at 0 50%, rgba(0,0,0,.18), transparent), radial-gradient(farthest-side at 100% 50%, rgba(0,0,0,.18), transparent) 100% 0; background-repeat: no-repeat; background-size: 40px 100%, 40px 100%, 14px 100%, 14px 100%; background-attachment: local, local, scroll, scroll; }
.tablewrap.scrolls { padding-bottom: 18px; }
.tablewrap.scrolls::after { content: "swipe to see more →"; position: absolute; right: 0; bottom: 0; font: 10px/1 var(--f-mono); color: var(--muted); background: var(--card); padding: 3px 6px; border-radius: 4px; }

.exam { margin-top: 14px; border: 1.5px solid color-mix(in srgb, var(--good) 45%, transparent); background: var(--good-soft); border-radius: 8px; padding: 12px 14px; max-width: 70ch; }
.exam .eyebrow { color: var(--good); display: block; margin-bottom: 6px; }
.alt { margin-top: 14px; border-top: 1px solid var(--line); padding-top: 12px; display: flex; flex-direction: column; gap: 10px; max-width: 70ch; }
.alt-kinds { display: flex; gap: 6px; flex-wrap: wrap; }
.alt-kinds button { font: 700 12px var(--f-body); padding: 6px 11px; border-radius: 999px; border: 1.5px solid var(--line); background: var(--card); cursor: pointer; color: var(--muted); min-height: 32px; }
.alt-kinds button[aria-pressed="true"] { border-color: var(--mercury); color: var(--mercury); background: var(--mercury-soft); }
.alt-body { background: var(--mercury-soft); border-radius: 8px; padding: 12px 14px; display: flex; flex-direction: column; gap: 8px; animation: rise .25s ease; }
.alt-body .eyebrow { color: var(--mercury); }
.alt-body .reveal { margin-top: 4px; }
@keyframes rise { from { transform: translateY(4px); opacity: .4; } to { transform: none; opacity: 1; } }
.check { margin-top: 16px; border-top: 1px solid var(--line); padding-top: 14px; }
.check .eyebrow { display: block; margin-bottom: 8px; }
.quiz-sticky { position: sticky; bottom: calc(14px + env(safe-area-inset-bottom, 0px)); z-index: 4; display: flex; justify-content: flex-start; pointer-events: none; margin-top: 12px; }
.quiz-sticky .btn { pointer-events: auto; box-shadow: var(--shadow); }
@media (max-width: 760px) { .quiz-sticky { bottom: calc(76px + env(safe-area-inset-bottom, 0px)); } }

figure.dg { margin: 16px 0 0; display: flex; flex-direction: column; gap: 8px; }
figure.dg .dgbox { background: var(--paper); border: 1px solid var(--line); border-radius: 8px; padding: 10px; overflow-x: auto; position: relative; }
figure.dg svg { display: block; width: 100%; height: auto; max-width: 720px; margin: 0 auto; color: var(--ink); }
figure.dg figcaption { font-size: 13.5px; color: var(--muted); max-width: 70ch; }
.enlarge { position: absolute; top: 6px; right: 6px; z-index: 2; font: 700 11px var(--f-mono); padding: 5px 8px; border-radius: 6px; border: 1px solid var(--line); background: var(--card); color: var(--muted); cursor: pointer; min-height: 28px; }
.enlarge:hover { color: var(--ink); }
.overlay { position: fixed; inset: 0; z-index: 60; background: color-mix(in srgb, var(--paper) 96%, transparent); display: flex; flex-direction: column; padding-top: env(safe-area-inset-top, 0px); padding-bottom: env(safe-area-inset-bottom, 0px); padding-left: max(12px, env(safe-area-inset-left, 0px)); padding-right: max(12px, env(safe-area-inset-right, 0px)); overflow: auto; }
.overlay-bar { display: flex; justify-content: space-between; align-items: center; padding: 10px 4px; }
.overlay .dgbox { background: var(--paper); flex: 1; }
.overlay svg { max-width: 1100px; width: 100%; height: auto; display: block; margin: 0 auto; color: var(--ink); }
/* diagram classes, themed */
.d-ink { stroke: currentColor; fill: none; stroke-width: 2; }
.d-thin { stroke: currentColor; fill: none; stroke-width: 1; opacity: .5; }
.d-acc { stroke: var(--sodium); fill: none; stroke-width: 2.5; }
.d-acc2 { stroke: var(--mercury); fill: none; stroke-width: 2.5; }
.d-good { stroke: var(--good); fill: none; stroke-width: 2.5; }
.d-bad { stroke: var(--bad); fill: none; stroke-width: 2.5; }
.d-dash { stroke-dasharray: 5 4; }
.f-ink { fill: currentColor; stroke: none; }
.f-acc { fill: var(--sodium); stroke: none; }
.f-acc2 { fill: var(--mercury); stroke: none; }
.f-soft { fill: var(--sodium-soft); }
.f-soft2 { fill: var(--mercury-soft); }
.f-card { fill: var(--card); }
.f-paper { fill: var(--paper); }
.f-good { fill: var(--good); stroke: none; }
.f-bad { fill: var(--bad); stroke: none; }
.d-txt { fill: currentColor; font: 13px var(--f-body); }
.d-txt.s { font-size: 11px; }
.d-txt.b { font-weight: 700; }
.d-txt.m { font-family: var(--f-math); font-style: italic; font-size: 15px; }
.d-txt.acc { fill: var(--sodium); }
.d-txt.acc2 { fill: var(--mercury); }
.d-txt.mut { fill: var(--muted); }

/* sims */
.sim { display: flex; flex-direction: column; gap: 10px; }
.sim-controls { display: grid; grid-template-columns: repeat(auto-fit, minmax(min(100%, 200px), 1fr)); gap: 8px 16px; }
.slider { display: flex; flex-direction: column; gap: 2px; font-size: 13px; }
.slider label { display: flex; justify-content: space-between; gap: 8px; color: var(--muted); }
.slider output { font: 600 13px var(--f-mono); color: var(--ink); font-variant-numeric: tabular-nums; }
.slider input[type=range] { width: 100%; accent-color: var(--sodium); min-height: 28px; }
.readout { display: flex; flex-wrap: wrap; gap: 6px 16px; font: 13px/1.4 var(--f-mono); color: var(--muted); }
.readout b { color: var(--ink); font-weight: 600; }
.sim canvas { width: 100%; height: auto; display: block; border-radius: 6px; }

.listcard { padding: 18px 22px; margin-bottom: 16px; }
.listcard h2 { font-size: 20px; margin-bottom: 10px; }
.listcard ul { margin: 0; padding-left: 20px; display: flex; flex-direction: column; gap: 8px; max-width: 70ch; }
.mistake li::marker { color: var(--bad); }
.tips li::marker { color: var(--good); }

/* formula cards */
.flist { display: grid; grid-template-columns: repeat(auto-fill, minmax(min(100%, 300px), 1fr)); gap: 12px; }
.fcard { padding: 14px 16px; display: flex; flex-direction: column; gap: 8px; }
.fcard .eq { font-size: 24px; overflow-x: auto; white-space: nowrap; padding: 4px 0; }
.fcard .fname { font-weight: 700; }
.fcard .fvars { font-size: 13.5px; color: var(--muted); line-height: 1.5; }
.fcard .ftag { display: flex; gap: 6px; flex-wrap: wrap; align-items: center; }
.fcard .flink { margin-top: auto; padding-top: 8px; border-top: 1px solid var(--line); font-size: 13px; text-decoration: none; color: var(--muted); display: flex; min-height: 32px; align-items: center; }
.fcard .flink:hover { color: var(--sodium); }

/* flashcards */
.flash { perspective: 1200px; max-width: 620px; margin: 0 auto; }
.flash-card { position: relative; min-height: 240px; cursor: pointer; transform-style: preserve-3d; transition: transform .45s cubic-bezier(.2,.7,.2,1); }
.flash-card.flipped { transform: rotateY(180deg); }
.flash-face { position: absolute; inset: 0; backface-visibility: hidden; -webkit-backface-visibility: hidden; padding: 24px; display: flex; flex-direction: column; justify-content: center; gap: 10px; text-align: center; overflow: auto; }
.flash-face.back { transform: rotateY(180deg); }
.flash-face .q { font: 700 21px/1.3 var(--f-display); }
.flash-face .qsmall { font-size: 13px; color: var(--muted); }
.flash-face .a { font-size: 17px; }
.grade { display: grid; grid-template-columns: repeat(3, 1fr); gap: 8px; margin-top: 14px; max-width: 620px; margin-left: auto; margin-right: auto; }
.grade button { padding: 10px 6px; border-radius: 9px; border: 1.5px solid var(--line); background: var(--card); font-weight: 700; cursor: pointer; display: flex; flex-direction: column; align-items: center; gap: 2px; min-height: 52px; }
.grade button small { font: 11px var(--f-mono); color: var(--muted); font-weight: 400; }
.grade button.g0:hover { border-color: var(--bad); background: var(--bad-soft); }
.grade button.g1:hover { border-color: var(--sodium); background: var(--sodium-soft); }
.grade button.g2:hover { border-color: var(--good); background: var(--good-soft); }

/* quiz */
.qwrap { max-width: 720px; margin: 0 auto; }
.qcard { padding: 22px; display: flex; flex-direction: column; gap: 14px; }
.qprog { display: flex; gap: 3px; }
.qprog i { flex: 1; height: 5px; border-radius: 3px; background: var(--line); }
.qprog i.ok { background: var(--good); }
.qprog i.no { background: var(--bad); }
.qprog i.cur { background: var(--sodium); }
.qtext { font-size: 18.5px; line-height: 1.5; }
.qgraph { background: var(--paper); border: 1px solid var(--line); border-radius: 8px; padding: 8px; overflow-x: auto; }
.qgraph svg { width: 100%; height: auto; max-width: 560px; display: block; margin: 0 auto; color: var(--ink); }
.qopts { display: flex; flex-direction: column; gap: 8px; }
.qopt { text-align: left; padding: 12px 14px; border-radius: 9px; border: 1.5px solid var(--line); background: var(--card); cursor: pointer; display: flex; gap: 12px; align-items: flex-start; font-size: 16px; min-height: 44px; }
.qopt:hover:not(:disabled) { border-color: var(--ink); }
.qopt .k { font: 700 13px/1.6 var(--f-mono); color: var(--muted); flex: none; }
.qopt.right { border-color: var(--good); background: var(--good-soft); }
.qopt.wrong { border-color: var(--bad); background: var(--bad-soft); }
.qopt.idk { color: var(--muted); border-style: dashed; }
.qopt:disabled { cursor: default; }
.qnum { display: flex; gap: 8px; align-items: center; flex-wrap: wrap; }
.qnum input { font: 600 18px var(--f-mono); width: 180px; max-width: 100%; padding: 9px 12px; border-radius: 9px; border: 1.5px solid var(--line); background: var(--card); color: var(--ink); }
.qnum input:focus { border-color: var(--sodium); outline: none; }
.qnum .unit { font-family: var(--f-math); font-size: 18px; }
.verdict { border-radius: 9px; padding: 14px 16px; display: flex; flex-direction: column; gap: 8px; animation: rise .25s ease; }
.verdict.ok { background: var(--good-soft); }
.verdict.no { background: var(--bad-soft); }
.verdict.mid { background: var(--sodium-soft); }
.verdict h3 { font-size: 17px; }
.verdict.ok h3 { color: var(--good); }
.verdict.no h3 { color: var(--bad); }
.verdict .why { display: flex; flex-direction: column; gap: 6px; }
.parts { display: flex; flex-direction: column; gap: 14px; }
.part { border: 1px solid var(--line); border-radius: 9px; padding: 12px 14px; display: flex; flex-direction: column; gap: 8px; }
.part .ph { display: flex; justify-content: space-between; gap: 8px; align-items: baseline; }
.part .marks { font: 600 12px var(--f-mono); color: var(--muted); }
textarea.ans { width: 100%; min-height: 90px; font: inherit; padding: 10px 12px; border-radius: 9px; border: 1.5px solid var(--line); background: var(--paper); color: var(--ink); resize: vertical; }
textarea.ans:focus { border-color: var(--sodium); outline: none; }
.scheme { background: var(--good-soft); border-radius: 8px; padding: 10px 12px; display: flex; flex-direction: column; gap: 6px; }
.scheme .eyebrow { color: var(--good); }
.chk { display: flex; flex-direction: column; gap: 6px; margin: 0; padding: 0; list-style: none; }
.chk li { display: flex; gap: 10px; align-items: flex-start; }
.chk input { width: 20px; height: 20px; margin-top: 3px; accent-color: var(--good); flex: none; }
.chk label { cursor: pointer; }
.chk li.missed label { color: var(--bad); }
.setup { padding: 22px; display: flex; flex-direction: column; gap: 18px; }
.pickgroup { display: flex; flex-direction: column; gap: 8px; }
.pick { display: flex; flex-wrap: wrap; gap: 8px; }
.pick button { padding: 8px 12px; border-radius: 999px; border: 1.5px solid var(--line); background: var(--card); cursor: pointer; font-weight: 700; font-size: 14px; min-height: 36px; }
.pick button[aria-pressed="true"] { border-color: var(--sodium); background: var(--sodium-soft); }
.score { text-align: center; padding: 30px 22px; display: flex; flex-direction: column; gap: 12px; align-items: center; }
.score b.big { font: 800 72px/1 var(--f-display); letter-spacing: -0.04em; font-variant-numeric: tabular-nums; }
.stream { white-space: pre-wrap; font-size: 15.5px; }

/* feynman + experiments */
.fey { padding: 22px; display: flex; flex-direction: column; gap: 12px; margin-bottom: 16px; }
.fey textarea { width: 100%; min-height: 120px; font: inherit; padding: 12px; border-radius: 9px; border: 1.5px solid var(--line); background: var(--paper); color: var(--ink); resize: vertical; }
.fey textarea:focus { border-color: var(--sodium); outline: none; }
.fey .model { background: var(--good-soft); border-radius: 9px; padding: 12px 14px; display: flex; flex-direction: column; gap: 8px; }
.fey .common { background: var(--sodium-soft); border-radius: 8px; padding: 10px 12px; font-size: 14.5px; }
.expt-parts { display: grid; gap: 10px; }
.expt-part { display: flex; flex-direction: column; gap: 4px; }
.expt-part label { font: 700 12px/1.3 var(--f-mono); letter-spacing: .06em; text-transform: uppercase; color: var(--muted); }
.expt-part textarea { min-height: 70px; }
.expt-model { display: flex; flex-direction: column; gap: 12px; }
.expt-model h4 { font: 700 12px/1.3 var(--f-mono); letter-spacing: .06em; text-transform: uppercase; color: var(--muted); margin-bottom: 4px; }
.expt-model ol, .expt-model ul { margin: 0; padding-left: 20px; display: flex; flex-direction: column; gap: 4px; }

.empty { padding: 30px; text-align: center; color: var(--muted); display: flex; flex-direction: column; gap: 12px; align-items: center; }
.toast { position: fixed; left: 50%; bottom: calc(80px + env(safe-area-inset-bottom, 0px)); transform: translateX(-50%); background: var(--ink); color: var(--card); padding: 10px 16px; border-radius: 9px; font-weight: 700; font-size: 14px; z-index: 70; animation: rise .2s ease; max-width: calc(100vw - 32px); }
.foot { margin-top: 40px; font-size: 13px; color: var(--muted); max-width: 80ch; }
.examgrid { display: grid; grid-template-columns: repeat(auto-fit, minmax(min(100%, 280px), 1fr)); gap: 12px; }
.code { font: 13px/1.5 var(--f-mono); background: var(--paper); border: 1px solid var(--line); border-radius: 8px; padding: 10px; width: 100%; min-height: 90px; color: var(--ink); resize: vertical; }

/* account sheet + tutor panel */
.sheet { position: fixed; inset: auto 0 0 0; z-index: 65; background: var(--card); border-top: 1px solid var(--line); border-radius: 16px 16px 0 0; box-shadow: 0 -10px 40px -20px rgba(0,0,0,.5); padding-top: 12px; padding-bottom: calc(16px + env(safe-area-inset-bottom, 0px)); padding-left: max(16px, env(safe-area-inset-left, 0px)); padding-right: max(16px, env(safe-area-inset-right, 0px)); max-height: 85vh; display: flex; flex-direction: column; gap: 12px; animation: up .25s ease; }
@keyframes up { from { transform: translateY(20px); opacity: 0; } to { transform: none; opacity: 1; } }
@media (min-width: 761px) { .sheet { left: auto; right: 16px; bottom: 16px; width: min(460px, calc(100vw - 32px)); border-radius: 14px; border: 1px solid var(--line); max-height: min(80vh, 720px); } }
.sheet-head { display: flex; justify-content: space-between; align-items: center; gap: 8px; }
.sheet-head h3 { font-size: 18px; }
.sheet-body { overflow: auto; display: flex; flex-direction: column; gap: 10px; min-height: 0; }
.msg { padding: 10px 12px; border-radius: 10px; font-size: 15px; max-width: 95%; white-space: pre-wrap; }
.msg.me { align-self: flex-end; background: var(--sodium-soft); }
.msg.ai { align-self: flex-start; background: var(--paper); border: 1px solid var(--line); }
.msg.ai.err { border-color: var(--bad); color: var(--bad); }
.starters { display: flex; flex-wrap: wrap; gap: 6px; }
.starters button { font-size: 13px; padding: 6px 10px; border-radius: 999px; border: 1px solid var(--line); background: var(--card); cursor: pointer; color: var(--muted); min-height: 32px; text-align: left; }
.starters button:hover { color: var(--ink); border-color: var(--ink); }
.tutor-form { display: flex; gap: 8px; align-items: flex-end; }
.tutor-form textarea { flex: 1; min-height: 44px; max-height: 140px; font: inherit; padding: 10px 12px; border-radius: 9px; border: 1.5px solid var(--line); background: var(--paper); color: var(--ink); resize: none; }
.tutor-form textarea:focus { border-color: var(--sodium); outline: none; }
.tutor-fab { position: fixed; right: max(16px, env(safe-area-inset-right, 0px)); bottom: calc(18px + env(safe-area-inset-bottom, 0px)); z-index: 40; box-shadow: var(--shadow); border-radius: 999px; }
@media (max-width: 760px) { .tutor-fab { bottom: calc(78px + env(safe-area-inset-bottom, 0px)); } }
.usage { font: 11px var(--f-mono); color: var(--muted); }
.acct-opts { display: flex; flex-direction: column; gap: 10px; }
.signin { display: flex; flex-direction: column; gap: 8px; padding: 12px 14px; border-radius: 9px; background: var(--paper); border: 1px solid var(--line); }
.signin p { font-size: 15px; }
.signin .row { flex-wrap: nowrap; align-items: stretch; }
.signin input[type="email"] { flex: 1; min-width: 0; font: inherit; font-size: 16px; padding: 9px 12px; border-radius: 9px; border: 1.5px solid var(--line); background: var(--card); color: var(--ink); }
.signin input[type="email"]:focus { border-color: var(--sodium); outline: none; }
.signin .btn { flex: none; white-space: nowrap; }
.signin .hint.err { color: var(--bad); }
@media (max-width: 480px) { .signin .row { flex-wrap: wrap; } .signin .btn { flex: 1 1 100%; } }
@media (prefers-reduced-motion: reduce) { *, *::before, *::after { animation: none !important; transition: none !important; } }

/* Phones: give diagrams the width back. Sections and diagram boxes lose most of their
   padding, diagram boxes bleed into the section gutter, and the small label size goes
   from 11 to 12 px so labels on a 400-unit viewBox stay at or above 8.5 CSS px. */
@media (max-width: 480px) {
  .sec, .qcard, .listcard, .fey { padding-left: 14px; padding-right: 14px; }
  .sec { padding-top: 16px; padding-bottom: 16px; }
  figure.dg .dgbox { padding: 6px 4px; margin-left: -8px; margin-right: -8px; }
  .qgraph { padding: 6px 4px; margin-left: -8px; margin-right: -8px; }
  .check .qcard { padding-left: 0; padding-right: 0; }
  .d-txt { font-size: 13.5px; }
  .d-txt.s { font-size: 12px; }
}