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/dashboard.css

@@ -1392,97 +1392,14 @@ body.dashboard-shell.has-selection { padding-bottom: 4.5em; }
   cursor: default;
 }
 
-/* ─── ST4TS tab: hive-wide turn-stats rollup ──────────────────────────────
-   Plain tables + CSS bars (no chart lib in the dashboard bundle). */
-.hive-stats-windows {
-  display: flex;
-  flex-wrap: wrap;
-  gap: 6px;
-  margin: 12px 0;
-}
-.hive-stats-windows .btn.active {
-  border-color: var(--amber);
-  color: var(--amber);
-}
-.hive-stats-chips {
-  display: flex;
-  flex-wrap: wrap;
-  gap: 8px;
-  margin-bottom: 16px;
-}
-.hive-stats-chip {
-  display: inline-flex;
-  flex-direction: column;
-  padding: 6px 10px;
-  background: var(--bg-elev);
-  border: 1px solid var(--border);
-  border-radius: 4px;
-  min-width: 7em;
-}
-.hive-stats-chip .k {
-  font-size: 0.72rem;
-  color: var(--muted);
-  text-transform: lowercase;
-}
-.hive-stats-chip .v {
-  font-size: 1.05rem;
-  color: var(--fg);
-}
-.hive-stats-chip.est .v {
-  color: var(--amber);
-}
-.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;
-}
-.hive-stats-bar {
-  display: grid;
-  grid-template-columns: 12em 1fr 4em;
-  align-items: center;
-  gap: 8px;
-  margin: 3px 0;
-  font-size: 0.85rem;
-}
-.hive-stats-bar .track {
-  background: var(--bg-elev);
-  border: 1px solid var(--border);
-  border-radius: 3px;
-  height: 14px;
-  overflow: hidden;
-}
-.hive-stats-bar .fill {
-  display: block;
-  height: 100%;
-  background: var(--purple);
-}
-.hive-stats-bar .cnt {
-  text-align: right;
-  color: var(--muted);
-  font-variant-numeric: tabular-nums;
-}
+/* ST4TS moved to its own page (`/stats.html`): the
+   window selector / summary chips / bars live in stats.css, and the
+   shared `.hive-stats-table` moved to common.css (the SYST3M ›
+   C0NT41N3R L04D table below still uses it). */
 
 /* ─── SYST3M › C0NT41N3R L04D: live cgroup cpu/mem ─────────────────────────────
-   Reuses the `.hive-stats-table` styling from the ST4TS tab; only the
-   inline meter bar is new. */
+   Reuses the shared `.hive-stats-table` styling (now in common.css);
+   only the inline meter bar is new. */
 .cload-meter {
   display: inline-block;
   width: 6em;