feat(ui): add layout, club-switcher, and auth guard

Implements Task 18: App Layout + Club-Switcher + Auth Guard

New components:
- TenantContext: Manages activeClubId state with TanStack Query
- QueryProvider: TanStack Query client wrapper (60s stale time)
- AuthGuard: Auth + tenant redirect logic (unauthenticated → /login)
- ClubSwitcher: shadcn DropdownMenu for switching clubs
- SignOutButton: Simple sign out button
- Protected layout: Sidebar navigation + top bar with ClubSwitcher

Key features:
- Fetches clubs from /api/clubs/me
- Auto-loads activeClubId from localStorage
- Sets X-Tenant-Id cookie on club switch
- Invalidates all queries on club switch
- Redirect logic: unauthenticated → /login, 0 clubs → message, 1 club → auto-select, >1 clubs + no active → /select-club

TDD:
- 6 AuthGuard tests (loading, unauthenticated, 0 clubs, 1 club, multiple clubs, authenticated)
- 3 ClubSwitcher tests (renders current club, lists all clubs, calls setActiveClub on selection)

Dependencies:
- Added @tanstack/react-query

All tests pass (25/25). Build succeeds.

frontend/bun.lock

@@ -6,6 +6,7 @@
       "name": "frontend",
       "dependencies": {
         "@auth/core": "^0.34.3",
+        "@tanstack/react-query": "^5.90.21",
         "class-variance-authority": "^0.7.1",
         "clsx": "^2.1.1",
         "lucide-react": "^0.576.0",
@@ -543,6 +544,10 @@
 
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+    "@tanstack/query-core": ["@tanstack/query-core@5.90.20", "", {}, "sha512-OMD2HLpNouXEfZJWcKeVKUgQ5n+n3A2JFmBaScpNDUqSrQSjiveC7dKMe53uJUg1nDG16ttFPz2xfilz6i2uVg=="],
+
+    "@tanstack/react-query": ["@tanstack/react-query@5.90.21", "", { "dependencies": { "@tanstack/query-core": "5.90.20" }, "peerDependencies": { "react": "^18 || ^19" } }, "sha512-0Lu6y5t+tvlTJMTO7oh5NSpJfpg/5D41LlThfepTixPYkJ0sE2Jj0m0f6yYqujBwIXlId87e234+MxG3D3g7kg=="],
+
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