refactor(frontend): move S3TT1NGS to its own /settings.html page (#1464 step 2)

Extracts the operator-local preferences (browser-notification toggle)
out of the dashboard tab strip into a standalone /settings.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 settings.{html,css,js}; settings.js binds NOTIF + renders the
  shared server-warnings banner (matching the other standalone pages).
- drop the S3TT1NGS tab + pane from dashboard.html; the NOTIF.bind()
  toggle wiring moves to settings.js. The dashboard keeps NOTIF.show()
  (approvals / questions) — it reads the same browser permission +
  localStorage mute flag the settings page sets, so firing still works.
- move .notif-row / .btn-notif from dashboard.css to settings.css.
- add a Settings tile to the H0M3 hub; wire the new entries into
  build.mjs and update the nix/frontend.nix asset manifest comment.

First slice of #1464 step 2; the ST4TS page extraction follows separately.

frontend/packages/dashboard/src/index.html

@@ -39,6 +39,11 @@
         <span class="home-tile-desc">build · agent · system logs</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>
+      </a>
+
       <!-- Matrix tile: hidden until home.js confirms the matrix GUI is
            enabled (state.matrix_gui_enabled), mirroring the dashboard
            tab gating so operators without it don't see a dead link. -->