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/src/components/club-switcher.tsx

@@ -0,0 +1,54 @@
+'use client';
+
+import { useTenant } from '../contexts/tenant-context';
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuItem,
+  DropdownMenuLabel,
+  DropdownMenuSeparator,
+  DropdownMenuTrigger,
+} from './ui/dropdown-menu';
+import { Button } from './ui/button';
+import { Badge } from './ui/badge';
+
+export function ClubSwitcher() {
+  const { activeClubId, clubs, setActiveClub } = useTenant();
+
+  const activeClub = clubs.find(c => c.id === activeClubId);
+
+  return (
+    <DropdownMenu>
+      <DropdownMenuTrigger asChild>
+        <Button variant="outline" className="flex items-center gap-2">
+          {activeClub ? (
+            <>
+              <span>{activeClub.name}</span>
+              <Badge variant="secondary" className="ml-2">
+                {activeClub.sportType}
+              </Badge>
+            </>
+          ) : (
+            'Select Club'
+          )}
+        </Button>
+      </DropdownMenuTrigger>
+      <DropdownMenuContent align="end" className="w-[200px]">
+        <DropdownMenuLabel>My Clubs</DropdownMenuLabel>
+        <DropdownMenuSeparator />
+        {clubs.map(club => (
+          <DropdownMenuItem
+            key={club.id}
+            onClick={() => setActiveClub(club.id)}
+            className="flex items-center justify-between cursor-pointer"
+          >
+            <span>{club.name}</span>
+            <Badge variant="outline" className="ml-2 text-xs">
+              {club.sportType}
+            </Badge>
+          </DropdownMenuItem>
+        ))}
+      </DropdownMenuContent>
+    </DropdownMenu>
+  );
+}