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/app/(protected)/layout.tsx

@@ -0,0 +1,53 @@
+import { AuthGuard } from '@/components/auth-guard';
+import { ClubSwitcher } from '@/components/club-switcher';
+import Link from 'next/link';
+import { Button } from '@/components/ui/button';
+import { LogOut } from 'lucide-react';
+import { SignOutButton } from '@/components/sign-out-button';
+
+export default function ProtectedLayout({
+  children,
+}: {
+  children: React.ReactNode;
+}) {
+  return (
+    <AuthGuard>
+      <div className="flex min-h-screen bg-gray-50">
+        <aside className="hidden md:flex flex-col w-64 bg-white border-r">
+          <div className="p-4 border-b">
+            <h1 className="text-xl font-bold">WorkClub</h1>
+          </div>
+          <nav className="flex-1 p-4 space-y-2">
+            <Link href="/dashboard" className="flex items-center px-4 py-2 text-sm font-medium rounded-md hover:bg-gray-100">
+              Dashboard
+            </Link>
+            <Link href="/tasks" className="flex items-center px-4 py-2 text-sm font-medium rounded-md hover:bg-gray-100">
+              Tasks
+            </Link>
+            <Link href="/shifts" className="flex items-center px-4 py-2 text-sm font-medium rounded-md hover:bg-gray-100">
+              Shifts
+            </Link>
+            <Link href="/members" className="flex items-center px-4 py-2 text-sm font-medium rounded-md hover:bg-gray-100">
+              Members
+            </Link>
+          </nav>
+        </aside>
+
+        <div className="flex-1 flex flex-col">
+          <header className="bg-white border-b h-16 flex items-center justify-between px-6">
+            <div className="flex items-center gap-4">
+              <ClubSwitcher />
+            </div>
+            <div className="flex items-center gap-4">
+              <SignOutButton />
+            </div>
+          </header>
+
+          <main className="flex-1 p-6 overflow-auto">
+            {children}
+          </main>
+        </div>
+      </div>
+    </AuthGuard>
+  );
+}