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/dashboard.html

@@ -78,15 +78,9 @@
            into the log viewer (an agent's logs, a build entry's log)
            still live inside the relevant dashboard content. -->
 
-      <!-- S3TT1NGS: lock-bump + matrix-gui config. A regular tab in the
-           strip now (the responsive overflow ⋮ menu was dropped — if the
-           strip ever runs out of room we move panes to their own pages
-           rather than hide them). -->
-      <a class="tab" id="tab-settings" href="#settings" role="tab"
-         aria-controls="tab-pane-settings"
-         data-tab="settings">
-        <span class="tab-label">◆ S3TT1NGS ◆</span>
-      </a>
+      <!-- S3TT1NGS lives on its own page now (`/settings.html`), reached
+           from the H0M3 hub — operator-local prefs don't need a slot in
+           the operational tab strip. -->
     </nav>
   </header>
 
@@ -282,26 +276,11 @@
          tabs.js boots on this page the corresponding renderers
          no-op silently (each guard is `if (!el) return`). -->
 
-    <!-- S3TT1NGS: operator-local preferences. Today this is just the
-         browser-notification toggle (per-browser localStorage state).
-         The same IDs that used to live in the tab strip migrated
-         here unchanged so `NOTIF.bind()` in common.js wires them
-         without any JS change. -->
-    <section class="tab-pane" id="tab-pane-settings" data-tab-pane="settings" hidden
-             role="tabpanel" aria-labelledby="tab-settings">
-      <h2>◆ S3TT1NGS ◆</h2>
-      <div class="divider">══════════════════════════════════════════════════════════════</div>
-      <p class="meta">operator-local preferences. these live in the browser's localStorage — they do not sync between devices and do not survive a profile wipe.</p>
-
-      <h3>◇ browser notifications</h3>
-      <p class="meta">desktop notifications for new approvals, new operator questions, and broker messages addressed to you. requires a secure context (https or localhost). mute silences the notifications without revoking the OS-level permission.</p>
-      <div id="notif-row" class="notif-row">
-        <button type="button" id="notif-enable" class="btn btn-notif" hidden>🔔 enable notifications</button>
-        <button type="button" id="notif-mute"   class="btn btn-notif" hidden>🔕 mute</button>
-        <button type="button" id="notif-unmute" class="btn btn-notif" hidden>🔔 unmute</button>
-        <span id="notif-status" class="meta" hidden></span>
-      </div>
-    </section>
+    <!-- S3TT1NGS: operator-local preferences (browser-notification
+         toggle) live on their own page now (`/settings.html`), reached
+         from the H0M3 hub. The notif buttons moved there verbatim;
+         `NOTIF.show()` on this page still fires (it reads the same
+         localStorage + browser permission the settings page sets). -->
 
   </main>