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/flow.html

@@ -38,17 +38,15 @@
         <span class="tab-label">◆ FL0W ◆</span>
         <span class="tab-count" id="tab-count-flow" hidden></span>
       </a>
-
-      <!-- Notif controls cohabit with the tabs (always-on chrome).
-           Same IDs as on the dashboard so the shared NOTIF binding
-           (from common.js, imported by both tabs.js and flow.js)
-           picks them up unchanged. -->
-      <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 cross-links to the dashboard tab — notification
+           toggle + future preferences live there now. `NOTIF.show()`
+           in common.js doesn't depend on the buttons existing here,
+           so notifications still fire on this page when they're
+           enabled (or muted) in dashboard settings. -->
+      <a class="tab tab-link" href="/#settings"
+         title="open S3TT1NGS on the dashboard">
+        <span class="tab-label">◆ S3TT1NGS ◆ →</span>
+      </a>
     </nav>
   </header>