feat(dashboard): add standalone C0R3 page (system tab → its own page)

Carves the dashboard's SYST3M tab content out into a standalone page at
/core.html, reached from the H0M3 hub, with a createTabStrip sub-tab
strip (default = Rebuild Queue): rebuild queue, meta inputs, kept state,
container load. Same minimal-chrome standalone-page pattern as
/logs.html (← home back-link + sub-tab nav).

This is the additive first step: the new page is its own esbuild bundle
that cold-loads /api/state and subscribes to /dashboard/stream for the
same live rebuild_queue_changed / meta_inputs_changed / meta_update_running
/ tombstones_changed events the dashboard uses. The four section
renderers are ported from tabs.js; the dashboard's SYST3M tab is left in
place and untouched, so this PR cannot regress the existing dashboard.
Removing the now-duplicate SYST3M tab + de-duplicating the renderers
(into a shared module) + splitting the shared section CSS out of
dashboard.css is the deliberate follow-up.

core.css imports dashboard.css wholesale (transitional) so the ported
sections render identically; dashboard.css does not import common.css so
nothing double-loads. build.mjs gains core.js / core.css / core.html
entries; no Rust change (hive-c0re serves dist/ via ServeDir, and the
/dashboard/stream + /dashboard/history routes are registered ahead of
the fallback).

Page name "C0R3" is a placeholder pending the operator's pick — trivially
renamed (the /core.html URL + the C0R3 label).

frontend/packages/dashboard/src/index.html

@@ -68,6 +68,14 @@
         <span class="home-tile-desc">operator-local prefs · browser notifications</span>
       </a>
 
+      <a class="home-tile" href="/core.html">
+        <span class="home-tile-head">
+          <span class="home-tile-icon" aria-hidden="true">⬡</span>
+          <span class="home-tile-label">Core</span>
+        </span>
+        <span class="home-tile-desc">rebuild queue · meta inputs · kept state · container load</span>
+      </a>
+
       <!-- Forge tile: hidden until home.js confirms the hive-forge
            container is up (state.forge_present); home.js also fills the
            href from state.forge_public_url (or the :3000 fallback), so