feat(unit-4): Dashboard — availability widget with TanStack Query

- Add @tanstack/react-query 5.101.0; wrap app with QueryClientProvider
- Add AvailabilityStatus type and AvailabilityResponse to api/types.ts
- Implement useAvailabilityStatus (staleTime 30s, stale-on-error preserved)
- Add AvailabilityStatusBadge with green/amber/red states and stale indicator
- Replace DashboardPage placeholder card with live availability widget
- Add MSW availability handler; update test/utils with QueryClientProvider
- 55/55 tests pass (FR-05)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/i18n/locales/nl/translation.json

@@ -59,8 +59,16 @@
     },
     "dashboard": {
         "title": "Dashboard",
-        "welcome": "Welkom terug, {{name}}",
-        "placeholder": "Je dashboard-widgets verschijnen hier."
+        "welcome": "Welkom terug, {{name}}"
+    },
+    "availability": {
+        "title": "Systeemstatus",
+        "available": "Beschikbaar",
+        "maintenance": "Onderhoud",
+        "unavailable": "Niet beschikbaar",
+        "errorTitle": "Status kon niet worden geladen",
+        "staleLabel": "Laatste bekende status",
+        "retry": "Opnieuw proberen"
     },
     "userMenu": {
         "language": "Taal",