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/tabs.js

@@ -3219,13 +3219,13 @@ window.marked = marked;
   })();
 
   // ─── tab routing ───────────────────────────────────────────────────────
-  // Hash-based: `#swarm` / `#call` / `#system` activate the matching
-  // pane on the dashboard. Empty hash defaults to SW4RM. FL0W is NOT
-  // a tab — it's a separate page (`/flow.html`) reached via the
-  // tab-strip link. Tab routing only applies when the tab DOM is
-  // present (e.g. not on the flow page itself, where these elements
-  // don't exist and the loop no-ops).
-  const TABS = ['swarm', 'call', 'system', 'schedules'];
+  // Hash-based: `#swarm` / `#call` / `#system` / `#schedules` /
+  // `#settings` activate the matching pane on the dashboard. Empty
+  // hash defaults to SW4RM. FL0W is NOT a tab — it's a separate page
+  // (`/flow.html`) reached via the tab-strip link. Tab routing only
+  // applies when the tab DOM is present (e.g. not on the flow page
+  // itself, where these elements don't exist and the loop no-ops).
+  const TABS = ['swarm', 'call', 'system', 'schedules', 'settings'];
   function activateTab(name) {
     const target = TABS.includes(name) ? name : TABS[0];
     for (const t of TABS) {