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/mocks/availability/handlers.ts

@@ -0,0 +1,11 @@
+import { http, HttpResponse } from 'msw';
+
+export const availabilityHandlers = [
+    http.get('*/Availability/status', () =>
+        HttpResponse.json({
+            status: 'Available',
+            checkedAt: new Date().toISOString(),
+            message: '',
+        }),
+    ),
+];