feat(realtime): global guest-order alert on the dashboard

Guest orders from the QR/digital menu already notified via SignalR, but only
screens that were open (KDS/POS/tables) reacted — and silently (a data refresh,
no alert). So staff on any other screen never knew a menu order arrived.

- Add a global useOrderAlerts() mounted in the dashboard shell: connects to
  /hubs/kds, joins the café group, and on a new GUEST order plays a chime + shows
  a toast (localized fa/en/ar) + nudges order/KDS/POS lists to refresh — on every
  screen.
- Filter to guest QR-menu orders only (not staff POS orders): LiveOrderDto now
  carries Source, set in MapLiveOrder (+ the delivery/snappfood mappers).

86 API tests pass; dashboard tsc + build clean.

web/dashboard/src/app/[locale]/(dashboard)/layout.tsx

@@ -9,6 +9,7 @@ import { RouteGuard } from "@/components/auth/route-guard";
 import { CafeThemeProvider } from "@/components/theme/cafe-theme-provider";
 import { useAuthStore } from "@/lib/stores/auth.store";
 import { useOfflineSync } from "@/lib/offline/use-offline-sync";
+import { useOrderAlerts } from "@/lib/realtime/use-order-alerts";
 
 export default function DashboardLayout({
   children,
@@ -20,6 +21,7 @@ export default function DashboardLayout({
   const user = useAuthStore((s) => s.user);
   const hasHydrated = useAuthStore((s) => s._hasHydrated);
   useOfflineSync(); // register online/offline listeners + load queue count
+  useOrderAlerts(); // global sound+toast alert for guest QR-menu orders, any screen
 
   useEffect(() => {
     // Wait for Zustand to finish reading localStorage before deciding to redirect.