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.html

@@ -60,14 +60,9 @@
         <span class="tab-count" id="tab-count-schedules" hidden></span>
       </a>
 
-      <!-- ST4TS: hive-wide turn-stats rollup (swarm totals, busiest
-           agents, model mix, est cost). Fetched on tab activation +
-           window change from GET /api/stats-hive. -->
-      <a class="tab" id="tab-stats" href="#stats" role="tab"
-         aria-controls="tab-pane-stats"
-         data-tab="stats">
-        <span class="tab-label">◆ ST4TS ◆</span>
-      </a>
+      <!-- ST4TS lives on its own page now (`/stats.html`), reached from
+           the H0M3 hub — the hive-wide rollup is a standalone read-only
+           view, not part of the operational tab strip. -->
 
       <!-- (Peer hives are no longer a tab — they render as a headline
            under SW4RM. See tab-pane-swarm.) -->
@@ -241,35 +236,11 @@
       </div>
     </section>
 
-    <!-- ST4TS: hive-wide turn-stats aggregate. Plain tables/bars (the
-         dashboard bundle has no chart lib); data from GET
-         /api/stats-hive?window=, fetched on tab activation + on
-         window change. Per-agent trend charts stay on each agent's
-         own /stats page. -->
-    <section class="tab-pane" id="tab-pane-stats" data-tab-pane="stats" hidden
-             role="tabpanel" aria-labelledby="tab-stats">
-      <h2>◆ ST4TS ◆</h2>
-      <div class="divider">══════════════════════════════════════════════════════════════</div>
-      <p class="meta">hive-wide turn statistics, aggregated across every agent over the selected window. <strong>cost is a rough estimate</strong> from approximate per-model list prices — it drifts and is a ballpark, not a bill.</p>
-      <div class="hive-stats-windows" id="hive-stats-windows">
-        <button type="button" class="btn" data-w="1h">1h</button>
-        <button type="button" class="btn" data-w="4h">4h</button>
-        <button type="button" class="btn active" data-w="24h">24h</button>
-        <button type="button" class="btn" data-w="3d">3d</button>
-        <button type="button" class="btn" data-w="7d">7d</button>
-        <button type="button" class="btn" data-w="30d">30d</button>
-      </div>
-      <div class="hive-stats-chips" id="hive-stats-summary"></div>
-      <h3>◇ busiest agents</h3>
-      <div id="hive-stats-agents"><p class="meta">loading…</p></div>
-      <h3>◇ model mix (turns across the swarm)</h3>
-      <div id="hive-stats-models"></div>
-      <!-- "favorite tools": most-run bash commands across the swarm.
-           Header + list hidden until the hive-bash-mcp capture has
-           recorded data, so the section never shows an empty block. -->
-      <h3 id="hive-stats-bash-h" hidden>◇ favorite tools (bash commands across the swarm)</h3>
-      <div id="hive-stats-bash" hidden></div>
-    </section>
+    <!-- ST4TS: hive-wide turn-stats aggregate lives on its own page now
+         (`/stats.html`), reached from the H0M3 hub. The markup + the
+         hive-stats render JS moved there; when tabs.js boots on the
+         dashboard the renderers are simply gone (no stats tab to
+         activate). -->
 
     <!-- FL0W: lives on its own page now (`/flow.html`). The
          message-flow + inbox + compose DOM only exists there — when