dashboard: new S3TT1NGS tab housing the notification toggle (closes #763)

Adds a fifth tab (◆ S3TT1NGS ◆) at the end of the dashboard tab
strip; moves the browser-notification controls out of the chrome
header into the new pane.

### Why

The chrome was carrying the notification controls as 'always-on
ergonomics' but the surface they belong on is operator preferences,
not navigation. With #589 / #701 likely adding more per-browser
preferences later (theme / density / etc.), having a dedicated
S3TT1NGS pane gives them a home so the chrome strip doesn't keep
accreting controls.

### Shape

- index.html: new `#tab-pane-settings` section with H2 + intro +
  H3 'browser notifications' group; same notif-row DOM (same IDs)
  migrated from the chrome
- flow.html: drops the duplicated notif-row from its chrome (FL0W
  reused the dashboard's strip); adds a `◆ S3TT1NGS ◆ →`
  cross-page link to `/#settings`. Notifications still fire on
  FL0W when enabled — NOTIF.show() reads
  Notification.permission + the hyperhive.notify.muted localStorage
  key directly, no buttons needed in the page DOM.
- tabs.js: adds 'settings' to TABS array so hash routing activates
  the pane
- dashboard.css: drops the chrome-anchoring `.tabbar #notif-row`
  rule and the flow-page reuse comment, both obsolete; the general
  `.notif-row` styling stays
- docs/web-ui.md: new `### S3TT1NGS tab` section; Chrome-header
  bullet drops 'Notification controls' + adds S3TT1NGS; Browser
  notifications section pointers at the new pane

### No JS or backend changes

Same IDs (notif-row, notif-enable, notif-mute, notif-unmute,
notif-status) so `NOTIF.bind()` in common.js wires the new pane
unchanged. NOTIF.show() dispatch is DOM-independent.

### Validation

`npm run --workspace @hive/dashboard build` clean (tabs.js 167.8kb,
flow.js 37.4kb, dashboard.css 50.2kb).

frontend/packages/dashboard/src/index.html

@@ -69,14 +69,15 @@
         <span class="tab-count" id="tab-count-flow" hidden></span>
       </a>
 
-      <!-- Notification controls live in the chrome (always-on
-           ergonomics; not tab-specific). -->
-      <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>
+      <!-- S3TT1NGS: operator-local preferences (browser notification
+           toggle today; future: theme, density). Sits at the end of
+           the strip because it's rarely-touched and not a primary
+           surface. See docs/web-ui.md::S3TT1NGS tab. -->
+      <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>
     </nav>
   </header>
 
@@ -178,6 +179,27 @@
          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"
+             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>
+
   </main>
 
   <footer>