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/dashboard.css

@@ -98,15 +98,6 @@ body.dashboard-shell {
   color: var(--red);
 }
 
-/* Notification controls cohabit with the tabs (always-on chrome). */
-.tabbar #notif-row {
-  margin-left: auto;
-  display: flex;
-  gap: 0.5em;
-  align-items: center;
-  padding-right: 0.5em;
-}
-
 /* Tab pane visibility — show only the active one. The .tab-pane-active
    class is set by tabs.js based on the URL hash; default (no hash)
    resolves to SW4RM. */
@@ -1582,10 +1573,6 @@ body.flow-shell .tabbar .tab.active.tab-link {
   border-color: var(--purple-dim);
   box-shadow: 0 -2px 12px -4px rgba(203, 166, 247, 0.4);
 }
-/* `.notif-row` styling lives under the shared `.tabbar #notif-row`
-   selector earlier in the file — the flow page reuses the dashboard
-   tab strip rather than carrying its own title/hint/back chrome. */
-
 /* Inbox pill — operator inbox flyout trigger. Sits right under the
    header so it stays in the operator's gaze without crowding the
    chat. Same shape as the agent page's header pills. */