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.

nix/frontend.nix

@@ -17,9 +17,9 @@
 #                     frontend/packages/dashboard/build.mjs):
 #       index.html (H0M3 hub, served at /) dashboard.html (operator SPA,
 #       served at /dashboard.html) flow.html logs.html settings.html
-#       favicon.svg
-#       static/{home,tabs,flow,logs,settings,stream-worker}.js{,.map}
-#       static/{colors,theme,common,home,dashboard,flow,logs,settings}.css
+#       stats.html favicon.svg
+#       static/{home,tabs,flow,logs,settings,stats,stream-worker}.js{,.map}
+#       static/{colors,theme,common,home,dashboard,flow,logs,settings,stats}.css
 #   $out/agent/       the per-agent default UI (layered with
 #                     hyperhive.frontend.extraFiles at activation time)
 #       index.html stats.html screen.html