feat(ui): add login page, club picker, and dashboard

Implements Task 21: Login Page + First-Login Club Picker + Dashboard

New pages:
- Login page: Sign in with Keycloak button, clean centered layout
- Select club page: Club selection cards for multi-club users
- Dashboard: Summary with task/shift counts and quick links

Key features:
- Login delegates to Keycloak via NextAuth signIn('keycloak')
- Club picker shows cards with name + sport type
- Clicking club → setActiveClub() → redirects to /dashboard
- Dashboard shows active club name, open tasks count, upcoming shifts count
- Quick action links to /tasks and /shifts pages
- TanStack Query hooks with proper filters (status: 'Open', future shifts only)

TDD:
- 2 login tests (component export, signIn exists)
- 2 select-club tests (component export, useTenant exists)
- 4 dashboard tests (component export, hooks exist)

Task 21 tests: 8/8 pass. Build succeeds (9 routes registered).

Note: 38 pre-existing tests from Tasks 10,18-20 fail due to Bun+vitest
test infrastructure issues (vi.mocked() not supported, localStorage mock).
This is technical debt to be addressed separately.

frontend/src/app/login/page.tsx

@@ -0,0 +1,26 @@
+'use client';
+
+import { signIn } from 'next-auth/react';
+import { Card, CardHeader, CardTitle, CardContent } from '@/components/ui/card';
+import { Button } from '@/components/ui/button';
+
+export default function LoginPage() {
+  const handleSignIn = () => {
+    signIn('keycloak');
+  };
+
+  return (
+    <div className="flex items-center justify-center min-h-screen bg-gray-50">
+      <Card className="w-96">
+        <CardHeader>
+          <CardTitle className="text-2xl text-center">WorkClub Manager</CardTitle>
+        </CardHeader>
+        <CardContent>
+          <Button onClick={handleSignIn} className="w-full">
+            Sign in with Keycloak
+          </Button>
+        </CardContent>
+      </Card>
+    </div>
+  );
+}