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/aidlc-state.md

@@ -5,7 +5,7 @@
 - **Feature Slug**: cms-frontend
 - **Project Type**: Brownfield
 - **Start Date**: 2026-06-16T20:27:00Z
-- **Current Stage**: CONSTRUCTION - Unit 3: Functional Design
+- **Current Stage**: CONSTRUCTION - Unit 4: Functional Design
 - **Branch**: unknown
 
 ## Workspace State
@@ -49,7 +49,7 @@
 - [x] Unit 0 — Backend Prerequisites (CORS + httpOnly cookie) — COMPLETED 2026-06-19T11:45:00Z
 - [x] Unit 1 — Project Scaffold & Infrastructure — COMPLETED 2026-06-20T17:30:00Z
 - [x] Unit 2 — Authentication Pages — COMPLETED 2026-06-22 (29/29 tests pass, build ✅, runtime bugs fixed)
-- [ ] Unit 3 — Layout & Navigation
+- [x] Unit 3 — Layout & Navigation — COMPLETED 2026-06-22 (all commits merged, tests pass)
 - [ ] Unit 4 — Dashboard
 - [ ] Unit 5 — User Management
 - [ ] Unit 6 — Profile, Settings & CMS Placeholder