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/stats.css

@@ -0,0 +1,84 @@
+/* ─── /stats.html — hive-wide turn-stats rollup ────────────────────
+   Extracted from the dashboard ST4TS tab. Same minimal-
+   chrome pattern as /flow.html and /logs.html: the back-link header +
+   title (.logs-header / .logs-back / .logs-title) live in common.css.
+   `.hive-stats-table` also lives in common.css — it's shared with the
+   dashboard SYST3M › C0NT41N3R L04D table. Only the ST4TS-specific
+   window selector, summary chips, and bars are page-specific. */
+
+body.stats-shell {
+  margin: 0;
+  padding: 0;
+}
+
+.stats-main {
+  padding: 1.2em 1.5em 2em;
+}
+
+/* Window selector. Buttons keep the shared `.btn` pill look; the active
+   one is flagged by createTabStrip via `.hive-tab--active` (was a bespoke
+   `.active` toggle). */
+.hive-stats-windows {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 6px;
+  margin: 12px 0;
+}
+.hive-stats-windows .btn.hive-tab--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-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;
+}