Stats: GitHub-style watch-time heatmap

- api/stats gains by_day: local-date -> seconds (same watch-time
  heuristic as by_weekday)
- ?stats renders a Mon..Sun x week grid (30d window, matches the
  page header) with 4 intensity levels + tooltip per cell + legend;
  replaces the by-weekday bar chart the heatmap subsumes
- test_stats covers by_day (sums, slug-scoped, empty case)

static/app.js

@@ -482,24 +482,44 @@ function histBoard(items, limit = 0) {
   return [chips, grid];
 }
 
+function heatMap(byDay, days) {
+  const today = new Date(); today.setHours(12, 0, 0, 0);
+  const win = new Date(today); win.setDate(win.getDate() - (days - 1));
+  const start = new Date(win);
+  start.setDate(start.getDate() - (start.getDay() + 6) % 7);  // back to Monday
+  const max = Math.max(1, ...Object.values(byDay || {}));
+  const box = el("div", "heat");
+  for (let d = new Date(start); d <= today; d.setDate(d.getDate() + 1)) {
+    const k = `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, "0")}-${String(d.getDate()).padStart(2, "0")}`;
+    const v = (byDay || {})[k] || 0;
+    const c = el("i");
+    if (d < win) c.className = "none";
+    else if (v) {
+      c.className = "l" + Math.min(4, Math.ceil(v / max * 4));
+      c.title = k + " \u00b7 " + fmtMin(v / 60);
+    }
+    box.append(c);
+  }
+  const lg = el("div", "heat-lg");
+  lg.append(el("span", "muted", "less"));
+  for (const cls of ["", "l1", "l2", "l3", "l4"]) {
+    const s = el("i");
+    if (cls) s.className = cls;
+    lg.append(s);
+  }
+  lg.append(el("span", "muted", "more"));
+  return [box, lg];
+}
+
 async function showStats() {
   skel(5);
   const d = await api("/api/stats?days=30");
   main.innerHTML = "";
   main.append(el("h2", null, "Stats \u2014 last 30 days"));
   main.append(statsCards(d));
-  const WD = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];
-  const mx = Math.max(1, ...d.by_weekday);
-  const wk = el("div", "wk");
-  d.by_weekday.forEach((s, i) => {
-    const col = el("div", "wk-col");
-    const b = el("div", "wk-bar");
-    b.style.height = Math.max(3, Math.round(s / mx * 50)) + "px";
-    b.title = fmtMin(s / 60);
-    col.append(b, el("div", "wk-day", WD[i]));
-    wk.append(col);
-  });
-  main.append(wk);
+  const hm = el("section", null);
+  hm.append(el("h2", null, "Watch time by day"), ...heatMap(d.by_day, d.days));
+  main.append(hm);
   const ta = topAnime(d);
   if (ta) main.append(ta);
   if (d.top_searches.length) {