refactor(frontend): move ST4TS to its own /stats.html page (#1464 step 2)

Extracts the hive-wide turn-stats rollup out of the dashboard tab strip
into a standalone /stats.html page, reached from the H0M3 hub — same
minimal-chrome pattern as /flow.html and /logs.html. The dashboard tab
strip is now purely operational.

- new stats.{html,css,js}; stats.js holds the moved render JS and
  fetches /api/stats-hive on load + window change.
- migrate the window selector (#hive-stats-windows) from a bespoke
  data-w/.active toggle to the shared createTabStrip — now hash-routed
  (#1h / #24h / …) and deep-linkable, matching the per-agent /stats page.
- drop the ST4TS tab + pane from dashboard.html and the hive-stats render
  block + the stats->refreshHiveStats lazy-load from tabs.js.
- move the shared .hive-stats-table to common.css (the dashboard SYST3M >
  C0NT41N3R L04D table still uses it); the ST4TS-only window/chip/bar
  styles go to stats.css.
- add a Stats tile to the H0M3 hub (and drop the now-stale "stats" from
  the Dashboard tile desc); wire build.mjs + the nix/frontend.nix manifest.

Second slice of #1464 step 2; follows the /settings.html extraction.

frontend/packages/dashboard/src/common.css

@@ -558,3 +558,31 @@ body.home-shell #server-warnings {
   background: color-mix(in srgb, var(--red) 18%, transparent);
   border-bottom: 1px solid var(--red);
 }
+
+/* ─── stats table ─────────────────────────────────────────────────
+   Shared right-aligned numeric table. Used by /stats.html (busiest
+   agents) and the dashboard SYST3M › C0NT41N3R L04D table — moved here
+   from dashboard.css when ST4TS became its own page. */
+.hive-stats-table {
+  width: 100%;
+  border-collapse: collapse;
+  font-size: 0.85rem;
+  margin-bottom: 8px;
+}
+.hive-stats-table th,
+.hive-stats-table td {
+  border-bottom: 1px solid var(--border);
+  padding: 5px 8px;
+  text-align: right;
+}
+.hive-stats-table th:first-child,
+.hive-stats-table td:first-child {
+  text-align: left;
+}
+.hive-stats-table th {
+  color: var(--muted);
+  font-weight: normal;
+}
+.hive-stats-table td.num {
+  font-variant-numeric: tabular-nums;
+}