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/index.html

@@ -26,7 +26,7 @@
 
       <a class="home-tile" href="/dashboard.html">
         <span class="home-tile-label">Dashboard</span>
-        <span class="home-tile-desc">containers · approvals · system · stats</span>
+        <span class="home-tile-desc">containers · approvals · permissions · schedules · system</span>
       </a>
 
       <a class="home-tile" href="/flow.html">
@@ -39,6 +39,11 @@
         <span class="home-tile-desc">build · agent · system logs</span>
       </a>
 
+      <a class="home-tile" href="/stats.html">
+        <span class="home-tile-label">Stats</span>
+        <span class="home-tile-desc">hive-wide turn stats · cost · model mix</span>
+      </a>
+
       <a class="home-tile" href="/settings.html">
         <span class="home-tile-label">Settings</span>
         <span class="home-tile-desc">operator-local prefs · browser notifications</span>