:root{
  --paper:#F5F7FB; --grid:#E2E8F1; --ink:#1A2846; --muted:#5B6880; --line:#CBD4E2;
  --surface:#FFFFFF; --pen:#C4272D; --pen-wash:rgba(196,39,45,.08);
  --serif:"STIX Two Text","Cambria Math",Cambria,"Times New Roman",serif;
  --sans:"Instrument Sans",system-ui,-apple-system,"Segoe UI",Roboto,sans-serif;
  color-scheme:light;
}
@media (prefers-color-scheme: dark){
  :root:not([data-theme="light"]){
    --paper:#10151E; --grid:#1A2130; --ink:#E2E8F3; --muted:#8E9AB1; --line:#2B3446;
    --surface:#161C28; --pen:#FF6F6F; --pen-wash:rgba(255,111,111,.1); color-scheme:dark;
  }
}
:root[data-theme="dark"]{
  --paper:#10151E; --grid:#1A2130; --ink:#E2E8F3; --muted:#8E9AB1; --line:#2B3446;
  --surface:#161C28; --pen:#FF6F6F; --pen-wash:rgba(255,111,111,.1); color-scheme:dark;
}
:root{box-sizing:border-box;padding-top:env(safe-area-inset-top,0px);padding-bottom:env(safe-area-inset-bottom,0px)}
*,*::before,*::after{box-sizing:inherit}
html{height:100%;scroll-padding-top:env(safe-area-inset-top,0px)}
body{
  margin:0;min-height:100%;color:var(--ink);font-family:var(--sans);font-size:1rem;line-height:1.5;
  background-color:var(--paper);
  background-image:linear-gradient(var(--grid) 1px,transparent 1px),linear-gradient(90deg,var(--grid) 1px,transparent 1px);
  background-size:28px 28px;background-position:-1px -1px;
  -webkit-font-smoothing:antialiased;
}
button{font:inherit;color:inherit}
:focus-visible{outline:2px solid var(--pen);outline-offset:3px;border-radius:6px}
var{font-family:var(--serif);font-style:italic}

.app{max-width:600px;margin:0 auto;padding:0 20px 48px}
header{display:flex;align-items:center;justify-content:space-between;height:68px}
.logo{font-family:var(--serif);font-weight:600;font-size:1.75rem;letter-spacing:.06em;margin:0;line-height:1}
.logo i{color:var(--pen);letter-spacing:.02em}
.tools{display:flex;gap:4px}
.icon-btn{width:44px;height:44px;display:grid;place-items:center;background:none;border:0;border-radius:10px;cursor:pointer;color:var(--ink)}
.icon-btn:hover{background:var(--pen-wash)}
.icon-btn svg{width:22px;height:22px}

.games{display:flex;gap:6px;justify-content:center;margin:4px 0 0}
.game{border:1px solid var(--line);background:var(--surface);padding:7px 22px;border-radius:999px;cursor:pointer;font-size:.95rem;color:var(--muted);font-weight:500}
.game[aria-pressed="true"]{background:var(--ink);border-color:var(--ink);color:var(--paper)}
.modes{display:flex;gap:22px;justify-content:center;margin:12px 0 0}
.mode{border:0;background:none;padding:4px 2px;cursor:pointer;font-size:.92rem;color:var(--muted)}
.mode[aria-pressed="true"]{color:var(--ink);font-weight:600;text-decoration:underline;text-decoration-color:var(--ink);text-decoration-thickness:2px;text-underline-offset:6px}
.levels{display:flex;gap:6px;justify-content:center;margin:8px 0 0;flex-wrap:wrap}
.levels[hidden]{display:none}
.level{border:0;background:none;padding:4px 10px;border-radius:6px;cursor:pointer;color:var(--muted);font-size:.9rem}
.level[aria-pressed="true"]{color:var(--ink);text-decoration:underline;text-decoration-color:var(--pen);text-decoration-thickness:2px;text-underline-offset:5px}

.sheet{margin-top:28px;text-align:center}
.meta{display:flex;justify-content:center;align-items:center;gap:12px;margin:0;color:var(--muted);font-size:.92rem}
.pips{display:inline-flex;gap:3px;vertical-align:middle}
.pips span{width:7px;height:7px;border-radius:50%;border:1.5px solid var(--muted)}
.pips span.on{background:var(--pen);border-color:var(--pen)}
.prompt{font-family:var(--serif);font-size:1.15rem;margin:22px 0 0;color:var(--muted)}
.prompt var{color:var(--ink)}

.eq-wrap{overflow-x:auto;margin:10px -20px 0;padding:14px 20px 18px;scrollbar-width:thin}
.equation{font-family:var(--serif);font-size:3.25rem;line-height:1.25;white-space:nowrap;display:inline-block;letter-spacing:.005em}
.equation .eqs{padding:0 .28em}
.frac{display:inline-flex;flex-direction:column;vertical-align:middle;text-align:center;font-size:.78em;margin:0 .08em;line-height:1.12}
.frac .fn{padding:0 .18em .04em;border-bottom:.055em solid currentColor}
.frac .fd{padding-top:.04em}

.answer{display:flex;align-items:center;justify-content:center;gap:14px;margin-top:18px}
.answer[hidden]{display:none}
.xeq{font-family:var(--serif);font-size:2rem;white-space:nowrap}
.answer input{
  width:4.6ch;font-family:var(--serif);font-size:2rem;text-align:center;color:var(--ink);
  background:var(--surface);border:1.5px solid var(--line);border-radius:10px;padding:2px 6px;
}
.answer input:focus{outline:none;border-color:var(--ink);box-shadow:0 0 0 3px var(--pen-wash)}
.answer input.shake{animation:shake .32s}
@keyframes shake{25%{transform:translateX(-5px)}50%{transform:translateX(5px)}75%{transform:translateX(-3px)}}
.check{background:var(--ink);color:var(--paper);border:0;border-radius:10px;padding:12px 20px;font-weight:600;cursor:pointer;font-size:1rem}
.check:hover{filter:brightness(1.15)}
.msg{min-height:1.6em;margin:12px 0 0;color:var(--pen);font-size:.95rem}

.tries{display:flex;gap:16px;justify-content:center;flex-wrap:wrap;margin:6px 0 0;padding:0;list-style:none;font-family:var(--serif);font-size:1.3rem;color:var(--muted)}
.tries li{position:relative;padding:0 4px}
.tries li::after{content:"";position:absolute;left:-2px;right:-2px;top:52%;height:2px;background:var(--pen);transform:rotate(-14deg);border-radius:2px}

.status{display:flex;justify-content:center;gap:22px;flex-wrap:wrap;margin:26px 0 0;font-size:.92rem;color:var(--muted);font-variant-numeric:tabular-nums}
.status b{color:var(--ink);font-weight:600}
.reveal-btn{background:none;border:0;color:var(--muted);text-decoration:underline;text-underline-offset:3px;cursor:pointer;margin-top:14px;font-size:.88rem;padding:6px}
.reveal-btn[hidden]{display:none}
.reveal-btn.arm{color:var(--pen)}

/* Finished state */
.final{font-family:var(--serif);font-size:2.4rem;margin-top:16px;display:flex;justify-content:center;align-items:center;gap:.3em}
.final[hidden]{display:none}
.ringed{position:relative;display:inline-block;padding:0 .2em}
.ring{position:absolute;left:-.45em;right:-.45em;top:-.28em;bottom:-.22em;width:calc(100% + .9em);height:calc(100% + .5em);overflow:visible;pointer-events:none}
.ring path,.tick path{fill:none;stroke:var(--pen);stroke-width:2.6;stroke-linecap:round;stroke-linejoin:round;vector-effect:non-scaling-stroke}
.tick{width:.75em;height:.75em;margin-left:.15em;overflow:visible}
.final.lost .ringed{color:var(--muted)}
.animate .ring path,.animate .tick path{stroke-dasharray:1;stroke-dashoffset:1}
.animate .ring path{animation:draw .6s .1s cubic-bezier(.4,.1,.3,1) forwards}
.animate .tick path{animation:draw .3s .7s ease-out forwards}
@keyframes draw{to{stroke-dashoffset:0}}

.result{margin-top:26px;text-align:center}
.result[hidden]{display:none}
.verdict{font-family:var(--serif);font-size:1.5rem;margin:0}
.summary{margin:4px 0 0;color:var(--muted)}
.work{margin:28px auto 0;max-width:460px;background:var(--surface);border:1px solid var(--line);border-radius:14px;padding:18px 18px 20px;text-align:left}
.work h2{font-size:.95rem;font-weight:600;margin:0 0 10px;color:var(--muted)}
.steps-scroll{overflow-x:auto}
.steps{list-style:none;margin:0;padding:0;display:grid;grid-template-columns:1fr auto 1fr;column-gap:.5em;font-family:var(--serif);font-size:1.3rem;min-width:max-content}
.steps li{display:contents}
.steps .note{grid-column:1/-1;font-family:var(--sans);font-size:.82rem;color:var(--pen);text-align:center;margin:8px 0 2px}
.steps .l{text-align:right;white-space:nowrap}
.steps .r{text-align:left;white-space:nowrap}
.steps .frac{font-size:.75em}
.animate .steps li > *{opacity:0;animation:fadein .35s forwards}
@keyframes fadein{to{opacity:1}}

.actions{display:flex;gap:10px;justify-content:center;flex-wrap:wrap;margin-top:22px}
.ghost{background:var(--surface);border:1px solid var(--line);border-radius:10px;padding:11px 18px;cursor:pointer;font-weight:500}
.ghost:hover{border-color:var(--ink)}
.next{margin-top:18px;color:var(--muted);font-size:.92rem;font-variant-numeric:tabular-nums}

/* Archive */
.archive{margin-top:26px}
.archive[hidden],.sheet[hidden]{display:none}
.arch-list{list-style:none;margin:0;padding:0;background:var(--surface);border:1px solid var(--line);border-radius:14px;overflow:hidden}
.arch-list[hidden]{display:none}
.arch-list li+li{border-top:1px solid var(--line)}
.arch-row{display:grid;grid-template-columns:3.2em 1fr auto 7.5em;align-items:center;gap:12px;width:100%;padding:13px 16px;background:none;border:0;cursor:pointer;text-align:left;font-size:.95rem}
.arch-row:hover{background:var(--pen-wash)}
.arch-row .an{font-family:var(--serif);font-size:1.1rem}
.arch-row .as{text-align:right;color:var(--muted);font-size:.88rem}
.arch-row .as.won{color:var(--ink)}
.arch-row .as.lost{color:var(--pen)}
.arch-empty{text-align:center;color:var(--muted);background:var(--surface);border:1px solid var(--line);border-radius:14px;padding:28px 20px}
.arch-empty[hidden]{display:none}
.arch-empty p{margin:0 0 16px}
.back{background:none;border:0;color:var(--muted);cursor:pointer;font-size:.9rem;padding:6px 8px;margin:14px 0 -14px;text-decoration:underline;text-underline-offset:3px}
.back[hidden]{display:none}
@media (max-width:420px){.arch-row{grid-template-columns:2.6em 1fr auto 6.2em;gap:8px;padding:12px}}

/* Logo button */
.logo-btn{background:none;border:0;padding:0;margin:0;font:inherit;letter-spacing:inherit;color:inherit;cursor:pointer;line-height:1}

/* Hints */
.hints{margin:18px auto 0;max-width:460px;background:var(--surface);border:1px dashed var(--line);border-radius:14px;padding:12px 16px 14px;overflow-x:auto}
.hints[hidden]{display:none}
.hints .steps{font-size:1.15rem}
.hints .steps .note{margin-top:6px}
.hints .steps li.new > *{animation:fadein .35s both}
.hint-btn{display:inline-flex;flex-direction:column;align-items:center;gap:0;margin-top:16px;background:var(--surface);border:1px solid var(--line);border-radius:10px;padding:8px 18px;cursor:pointer;font-weight:500;line-height:1.3}
.hint-btn small{font-size:.75rem;color:var(--muted);font-weight:400}
.hint-btn:hover:not(:disabled){border-color:var(--ink)}
.hint-btn:disabled{cursor:default;opacity:.55}
.hint-btn[hidden]{display:none}

/* Formula book */
.book-dlg{max-width:520px}
.book h3{font-size:1rem;font-weight:600;margin:20px 0 4px}
.book h3:first-child{margin-top:0}
.book p{margin:0 0 8px;font-size:.95rem;max-width:62ch}
.book .ex{font-family:var(--serif);font-size:1.15rem;background:var(--paper);border-radius:8px;padding:8px 12px;margin:8px 0;line-height:1.9}
.book .why{display:inline-block;font-family:var(--sans);font-size:.78rem;color:var(--pen);margin:0 10px;line-height:1.3}
.book p var,.help p var{font-size:1.12em}
.book .eqn{white-space:nowrap}
.book .why::before{content:"\2192\00a0"}
.book table{border-collapse:collapse;width:100%;margin:6px 0 8px;font-size:.95rem}
.book th{text-align:left;font-weight:600;font-size:.85rem;color:var(--muted);padding:4px 8px;border-bottom:1px solid var(--line)}
.book td{padding:6px 8px;border-bottom:1px solid var(--line)}
.book td:first-child{font-family:var(--serif);font-size:1.1rem;width:30%}
.book .frac{font-size:.8em}

/* Lattice */
:root{--entry:#2457B8;--hint:#A86400;--rel:rgba(36,87,184,.10)}
@media (prefers-color-scheme: dark){:root:not([data-theme="light"]){--entry:#8DB2FF;--hint:#F2B45C;--rel:rgba(141,178,255,.14)}}
:root[data-theme="dark"]{--entry:#8DB2FF;--hint:#F2B45C;--rel:rgba(141,178,255,.14)}
.lat-view{margin-top:26px;text-align:center}
.lat-view[hidden]{display:none}
.q{color:var(--pen)}
.lat-eq{font-family:var(--serif);font-size:1.5rem;margin:8px 0 14px;letter-spacing:.02em;white-space:nowrap;font-variant-numeric:tabular-nums}
.lat-eq .e{color:var(--entry)}
.lat-eq .h{color:var(--hint)}
.lat-scroll{overflow-x:auto;margin:0 -20px;padding:4px 20px}
.lattice{--cs:min(58px,calc((100vw - 44px) / 6));display:grid;justify-content:center;margin:0 auto;font-family:var(--serif)}
.lattice.mini{--cs:46px;margin:12px auto}
.lc{width:var(--cs);height:var(--cs);position:relative;display:flex;align-items:center;justify-content:center}
.lc.times{font-size:calc(var(--cs) * .4);color:var(--muted)}
.lc.cell{background:var(--surface);border-right:1px solid var(--line);border-bottom:1px solid var(--line)}
.lc.cell.bl{border-left:2px solid var(--ink)}
.lc.cell.bt{border-top:2px solid var(--ink)}
.lc.cell.br{border-right:2px solid var(--ink)}
.lc.cell.bb{border-bottom:2px solid var(--ink)}
.lc.cell::before{content:"";position:absolute;inset:0;pointer-events:none;
  background:linear-gradient(to top left,transparent calc(50% - .7px),var(--muted) calc(50% - .7px),var(--muted) calc(50% + .7px),transparent calc(50% + .7px));opacity:.55}
.lc.fbox,.lc.pbox{margin:3px;width:calc(var(--cs) - 6px);height:calc(var(--cs) - 6px);border:1px solid var(--line);border-radius:9px;background:var(--surface)}
.ld{display:flex;align-items:center;justify-content:center;border:0;background:none;padding:0;font:inherit;color:var(--ink);border-radius:6px;line-height:1}
button.ld{cursor:pointer}
.ld.ht,.ld.hu{position:absolute;width:46%;height:46%;font-size:calc(var(--cs) * .34)}
.ld.ht{left:3%;top:3%}
.ld.hu{right:3%;bottom:3%}
.ld.fd{width:100%;height:100%;font-size:calc(var(--cs) * .46);font-weight:600}
.ld.pd{position:absolute;right:6%;bottom:6%;width:62%;height:62%;font-size:calc(var(--cs) * .4);font-weight:600}
.ld.empty{color:var(--pen);opacity:.8}
.ld.entered{color:var(--entry)}
.ld.hinted{color:var(--hint)}
.ld.revealed{color:var(--muted)}
.ld.wrong{color:var(--pen);background:var(--pen-wash);text-decoration:line-through;text-decoration-thickness:1.5px}
.ld.rel{background:var(--rel)}
.ld.sel{box-shadow:0 0 0 2px var(--entry);background:var(--rel);opacity:1}
.fbox .ld.sel,.fbox .ld.rel{border-radius:8px}
.carry{position:absolute;left:3px;top:3px;width:38%;height:38%;border:1px dashed var(--line);border-radius:5px;background:none;padding:0;font:inherit;font-size:calc(var(--cs) * .22);color:var(--muted);cursor:pointer;display:flex;align-items:center;justify-content:center}
.carry.sel{border:1.5px solid var(--entry);color:var(--entry)}
.carry.done{border:0;cursor:default}
.keypad{display:grid;grid-template-columns:repeat(6,1fr);gap:6px;max-width:340px;margin:18px auto 0}
.keypad[hidden],.lat-actions[hidden]{display:none}
.keypad button{height:46px;border:1px solid var(--line);border-radius:10px;background:var(--surface);font-family:var(--serif);font-size:1.35rem;cursor:pointer;color:var(--ink)}
.keypad button:active{background:var(--rel)}
.keypad .del{grid-column:6;grid-row:1 / span 2;height:auto;font-family:var(--sans);font-size:1.1rem}
.lat-actions{display:flex;gap:10px;justify-content:center;margin-top:6px}
.lat-actions .ghost:disabled{opacity:.55;cursor:default}
.lat-hints{max-width:460px;margin:16px auto 0;padding:12px 16px 12px 34px;text-align:left;background:var(--surface);border:1px dashed var(--line);border-radius:12px;font-size:.92rem;color:var(--hint)}
.lat-hints[hidden]{display:none}
.lat-hints li+li{margin-top:6px}
.lat-links{display:flex;flex-wrap:wrap;justify-content:center;gap:4px 16px;margin-top:10px}
.lat-links [hidden]{display:none}
.lat-work h2+.lat-steps{margin-top:0}
.lat-work h2:not(:first-child){margin-top:18px}
.lat-steps{margin:0;padding-left:20px;font-size:.92rem}
.lat-steps li+li{margin-top:6px}
.lat-note{font-size:.88rem;color:var(--muted);margin:0 0 8px}
.lat-diag{list-style:none;margin:0 0 10px;padding:0;font-size:.88rem;font-variant-numeric:tabular-nums}
.lat-diag li{display:grid;grid-template-columns:6.2em 1fr auto;gap:8px;padding:4px 0;border-bottom:1px solid var(--line)}
.lat-diag li span:nth-child(2){font-family:var(--serif);font-size:1rem}
.lat-diag li span:last-child{color:var(--muted)}
.stats-h{font-size:.95rem;font-weight:600;margin:4px 0 6px}
.stats-h+.figs{margin-top:4px}
@media (max-width:420px){.lat-eq{font-size:1.25rem}}

/* Dialogs */
dialog{border:1px solid var(--line);border-radius:16px;background:var(--surface);color:var(--ink);padding:22px 22px 24px;width:calc(100% - 32px);max-width:420px;max-height:calc(100% - 48px)}
dialog::backdrop{background:rgba(10,16,30,.45)}
.dlg-head{display:flex;justify-content:space-between;align-items:center;margin-bottom:12px}
.dlg-head h2{font-family:var(--serif);font-size:1.45rem;font-weight:600;margin:0}
.figs{display:grid;grid-template-columns:repeat(3,1fr);gap:16px 8px;text-align:center;margin:8px 0 22px}
.figs b{display:block;font-family:var(--serif);font-size:1.9rem;font-weight:400;line-height:1.1;font-variant-numeric:tabular-nums}
.figs span{font-size:.8rem;color:var(--muted)}
.dist h3{font-size:.95rem;font-weight:600;margin:0 0 10px}
.bar-row{display:grid;grid-template-columns:4.5em 1fr 3.5em;align-items:center;gap:10px;margin:6px 0;font-size:.88rem}
.bar{height:10px;border-radius:5px;background:var(--grid);overflow:hidden}
.bar i{display:block;height:100%;background:var(--ink);border-radius:5px;min-width:0}
.bar-row .v{text-align:right;color:var(--muted);font-variant-numeric:tabular-nums}
.help p{margin:0 0 12px;max-width:60ch}
.help .example{font-family:var(--serif);font-size:1.35rem;text-align:center;margin:16px 0}
.toast{position:fixed;left:50%;bottom:calc(24px + env(safe-area-inset-bottom,0px));transform:translateX(-50%);background:var(--ink);color:var(--paper);padding:10px 16px;border-radius:10px;font-size:.92rem;opacity:0;transition:opacity .2s;pointer-events:none}
.toast.show{opacity:1}
.copybox{width:100%;min-height:7em;margin-top:10px;font:inherit;font-size:.9rem;border:1px solid var(--line);border-radius:8px;padding:8px;background:var(--paper);color:var(--ink)}
.sr{position:absolute;width:1px;height:1px;overflow:hidden;clip:rect(0 0 0 0);white-space:nowrap}

@media (max-width:420px){
  .equation{font-size:2.5rem}
  .xeq,.answer input{font-size:1.75rem}
  .status{gap:14px}
}
@media (prefers-reduced-motion:reduce){
  .animate .ring path,.animate .tick path{animation:none;stroke-dashoffset:0}
  .animate .steps li > *{animation:none;opacity:1}
  .answer input.shake{animation:none}
}