:root {
  --bg: #0f1220; --panel: #181c30; --panel2: #1f2440; --line: #2c3254; --text: #e8eaf6; --muted: #9aa3c7;
  --accent: #7c8cff; --accent2: #5eead4; --good: #34d399; --bad: #f87171; --gold: #fbbf24;
  --d1: #5eead4; --d2: #86efac; --d3: #fde047; --d4: #fb923c; --d5: #f472b6;
}
* { box-sizing: border-box; }
html { color-scheme: dark; }
body { margin: 0; background: radial-gradient(1200px 600px at 10% -10%, #1b2150 0, transparent 60%), var(--bg);
  color: var(--text); font: 16px/1.5 system-ui, -apple-system, "Segoe UI", Roboto, sans-serif; min-height: 100vh; }
a { color: var(--accent); text-decoration: none; }
button { font: inherit; cursor: pointer; }
.wrap { max-width: 980px; margin: 0 auto; padding: 0 18px 60px; }

/* header */
header.top { position: sticky; top: 0; z-index: 5; backdrop-filter: blur(10px); background: rgba(15,18,32,.85); border-bottom: 1px solid var(--line); }
header.top .wrap { display: flex; align-items: center; gap: 18px; padding-top: 10px; padding-bottom: 10px; flex-wrap: wrap; }
.brand { font-weight: 800; font-size: 1.15rem; letter-spacing: .3px; color: var(--text); }
.brand b { color: var(--accent); }
nav { display: flex; gap: 4px; }
nav a { padding: 6px 12px; border-radius: 8px; color: var(--muted); }
nav a.active, nav a:hover { background: var(--panel2); color: var(--text); }
.spacer { flex: 1; }
.stat { display: flex; align-items: center; gap: 6px; font-weight: 600; }
.xp { min-width: 150px; }
.xp small { color: var(--muted); font-weight: 500; display: block; line-height: 1.1; }
.bar { height: 7px; background: var(--panel2); border-radius: 99px; overflow: hidden; margin-top: 3px; }
.bar > i { display: block; height: 100%; background: linear-gradient(90deg, var(--accent), var(--accent2)); border-radius: 99px; transition: width .6s; }
.link { background: none; border: 0; color: var(--muted); padding: 6px 8px; }
.link:hover { color: var(--text); }

h1 { font-size: 1.6rem; margin: 28px 0 4px; }
h2 { font-size: 1.15rem; margin: 26px 0 10px; }
.sub { color: var(--muted); margin: 0 0 18px; }

/* cards */
h2 + .grid { margin-top: 0; }
.grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(290px, 1fr)); gap: 14px; }
.card { background: var(--panel); border: 1px solid var(--line); border-radius: 14px; padding: 18px; }
a.card { display: block; color: inherit; transition: transform .15s, border-color .15s; }
a.card:hover { transform: translateY(-2px); border-color: var(--accent); }
.path-icon { font-size: 1.8rem; width: 48px; height: 48px; display: grid; place-items: center; border-radius: 12px; background: var(--panel2); color: var(--accent2); font-family: "KaTeX_Main", serif; }
.path-card h3 { margin: 10px 0 2px; }
.path-card p { margin: 0 0 12px; color: var(--muted); font-size: .92rem; }
.meta { display: flex; justify-content: space-between; color: var(--muted); font-size: .85rem; margin-top: 6px; }
.done-badge { color: var(--good); font-weight: 700; }

/* problems */
.problem { background: var(--panel); border: 1px solid var(--line); border-left: 5px solid var(--dc, var(--accent)); border-radius: 14px; padding: 18px 20px; margin-bottom: 16px; }
.problem.solved { border-color: var(--good); background: linear-gradient(0deg, rgba(52,211,153,.06), rgba(52,211,153,.06)), var(--panel); }
.problem.revealed { opacity: .85; }
.p-head { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; margin-bottom: 8px; }
.badge { font-size: .75rem; font-weight: 700; padding: 2px 9px; border-radius: 99px; background: var(--panel2); color: var(--dc); border: 1px solid var(--dc); text-transform: uppercase; letter-spacing: .5px; }
.pts { margin-left: auto; font-weight: 700; color: var(--gold); }
.p-title { font-weight: 700; }
.topic { color: var(--muted); font-size: .85rem; }
.statement { font-size: 1.08rem; margin: 10px 0 14px; white-space: pre-wrap; overflow-x: auto; }
.answer-row { display: flex; gap: 8px; flex-wrap: wrap; }
input[type=text], input[type=password] { flex: 1; min-width: 220px; background: #0e1126; border: 1px solid var(--line); border-radius: 10px; color: var(--text); padding: 10px 12px; font: 1rem ui-monospace, SFMono-Regular, Menlo, monospace; }
input:focus { outline: 2px solid var(--accent); outline-offset: 1px; }
.btn { background: var(--accent); color: #0b0e22; border: 0; border-radius: 10px; padding: 10px 16px; font-weight: 700; }
.btn:hover { filter: brightness(1.1); }
.btn:disabled { opacity: .5; cursor: default; }
.btn.ghost { background: var(--panel2); color: var(--text); border: 1px solid var(--line); font-weight: 600; }
.row-actions { display: flex; gap: 8px; margin-top: 10px; flex-wrap: wrap; align-items: center; }
.preview { min-height: 0; margin-top: 6px; color: var(--muted); font-size: 1.05rem; overflow-x: auto; }
.preview.err { color: var(--bad); font-size: .9rem; }
.hint { background: var(--panel2); border-radius: 10px; padding: 8px 12px; margin-top: 8px; font-size: .95rem; }
.hint b { color: var(--accent2); }
.feedback { margin-top: 10px; padding: 10px 12px; border-radius: 10px; font-weight: 600; }
.feedback.bad { background: rgba(248,113,113,.12); color: var(--bad); animation: shake .35s; }
.feedback.good { background: rgba(52,211,153,.12); color: var(--good); }
.feedback.warn { background: rgba(251,191,36,.12); color: var(--gold); }
.solution { margin-top: 12px; padding: 12px 14px; border-radius: 10px; background: #12162b; border: 1px dashed var(--line); white-space: pre-wrap; }
.solution .ans { font-size: 1.15rem; margin-bottom: 6px; white-space: normal; }
.tip { color: var(--muted); font-size: .85rem; margin-top: 6px; }
@keyframes shake { 20%,60% { transform: translateX(-6px); } 40%,80% { transform: translateX(6px); } }
@keyframes pop { 0% { transform: scale(.6); opacity: 0; } 60% { transform: scale(1.15); } 100% { transform: scale(1); opacity: 1; } }
.pop { animation: pop .5s; }

/* tables / charts */
table { width: 100%; border-collapse: collapse; }
th, td { padding: 10px 12px; text-align: left; border-bottom: 1px solid var(--line); }
th { color: var(--muted); font-weight: 600; font-size: .85rem; text-transform: uppercase; letter-spacing: .5px; }
tr.me td { background: rgba(124,140,255,.1); font-weight: 700; }
.tabs { display: flex; gap: 6px; margin-bottom: 14px; }
.tabs button { background: var(--panel2); border: 1px solid var(--line); color: var(--muted); padding: 6px 14px; border-radius: 99px; }
.tabs button.on { color: var(--text); border-color: var(--accent); }
.chart { display: flex; align-items: flex-end; gap: 6px; height: 150px; padding-top: 10px; }
.chart div { flex: 1; display: flex; flex-direction: column; justify-content: flex-end; align-items: center; height: 100%; font-size: .7rem; color: var(--muted); }
.chart div i { width: 100%; background: linear-gradient(0deg, var(--accent), var(--accent2)); border-radius: 5px 5px 0 0; min-height: 2px; }

/* auth */
.auth { max-width: 380px; margin: 12vh auto 0; }
.auth h1 { text-align: center; }
.auth form { display: grid; gap: 10px; }
.auth input { min-width: 0; }
.err-msg { color: var(--bad); min-height: 1.4em; font-size: .92rem; }
.swap { text-align: center; color: var(--muted); margin-top: 12px; }

#toast { position: fixed; left: 50%; bottom: 28px; transform: translate(-50%, 80px); background: var(--panel2); border: 1px solid var(--gold); color: var(--gold);
  padding: 12px 20px; border-radius: 12px; font-weight: 700; opacity: 0; transition: .35s; z-index: 20; max-width: 90vw; text-align: center; }
#toast.show { transform: translate(-50%, 0); opacity: 1; }
.loading { text-align: center; color: var(--muted); padding: 60px 0; }
.spinner { width: 28px; height: 28px; border: 3px solid var(--line); border-top-color: var(--accent); border-radius: 50%; margin: 0 auto 12px; animation: spin .8s linear infinite; }
@keyframes spin { to { transform: rotate(360deg); } }
@media (max-width: 600px) { .xp { min-width: 110px; } header.top .wrap { gap: 10px; } }