feat(dashboard): configurable notification sound, desktop popups & tab unread badge

Per-device notification preferences (localStorage) drive three new alert
channels in the dashboard shell, all fed by the existing SignalR
NotificationReceived events:

- Sound: 6 selectable procedural Web Audio chimes + volume, no asset files.
- Desktop/Windows popups via the Notification API, fired only when the tab
  is backgrounded (in-app toast covers the focused case).
- Unread count on the browser tab: (N) title prefix + numbered favicon badge.

useOrderAlerts is now the single orchestrator (sound + toast + desktop),
each gated by prefs; topbar feed enableToasts disabled to avoid double toasts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/dashboard/src/components/notifications/notification-center.tsx

@@ -22,8 +22,9 @@ export function NotificationCenter() {
   const [open, setOpen] = useState(false);
   const [selected, setSelected] = useState<CafeNotification | null>(null);
 
+  // Toasts/sounds/desktop popups are owned by useOrderAlerts (mounted once in the
+  // dashboard shell) so they fire exactly once; here we only need the bell feed.
   const { items, unreadCount, openNotification, markAllRead } = useNotificationsFeed({
-    enableToasts: true,
     limit: 20,
   });