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>

aidlc-docs/features/cms-frontend/construction/unit-4/functional-design/domain-entities.md

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+# Domain Entities — Unit 4: Dashboard
+
+## Entities
+
+### AvailabilityStatus (enumeration)
+
+A union type representing the three possible system states returned by the API.
+
+| Value | Meaning |
+|---|---|
+| `Available` | System is fully operational |
+| `Maintenance` | System is in planned maintenance mode |
+| `Unavailable` | System is not accessible |
+
+---
+
+### AvailabilityResponse (API response shape)
+
+Returned by `GET /api/v1/Availability/status`.
+
+| Field | Type | Description |
+|---|---|---|
+| `status` | `AvailabilityStatus` | Current system status |
+| `checkedAt` | `string` (ISO 8601) | Timestamp when status was last evaluated |
+| `message` | `string` | Optional human-readable reason or note (may be empty string) |
+
+---
+
+## Entity Relationship Diagram
+
+```mermaid
+classDiagram
+    class AvailabilityResponse {
+        +status: AvailabilityStatus
+        +checkedAt: string
+        +message: string
+    }
+    class AvailabilityStatus {
+        <<enumeration>>
+        Available
+        Maintenance
+        Unavailable
+    }
+    AvailabilityResponse --> AvailabilityStatus : status
+
+    style AvailabilityResponse fill:#4CAF50,stroke:#2E7D32,color:#000
+    style AvailabilityStatus fill:#2196F3,stroke:#0D47A1,color:#000
+```
+
+AvailabilityResponse holds a status (one of three enumeration values) plus a timestamp and message.
+
+---
+
+## Type Placement
+
+Both `AvailabilityStatus` and `AvailabilityResponse` are added to `frontend/src/api/types.ts` — the shared domain types file, consistent with `User`, `AuthResponse`, and `SetupStatus`.
+
+---
+
+## TanStack Query Dependency
+
+`@tanstack/react-query` is added as a new production dependency. A shared `QueryClient` is configured in `main.tsx` and provided via `QueryClientProvider`. This enables `staleTime`, background refetching, and persistent cached data across error states.