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/(protected)/dashboard/page.tsx

@@ -0,0 +1,58 @@
+'use client';
+
+import { useTenant } from '@/contexts/tenant-context';
+import { useShifts } from '@/hooks/useShifts';
+import { useTasks } from '@/hooks/useTasks';
+import { Card, CardHeader, CardTitle, CardContent } from '@/components/ui/card';
+import { Button } from '@/components/ui/button';
+import Link from 'next/link';
+
+export default function DashboardPage() {
+  const { activeClubId, clubs } = useTenant();
+  const activeClub = clubs.find(c => c.id === activeClubId);
+  
+  const { data: tasksData } = useTasks({ status: 'Open' });
+  const { data: shiftsData } = useShifts({ 
+    startDate: new Date().toISOString() 
+  });
+
+  const openTasksCount = tasksData?.total || 0;
+  const upcomingShiftsCount = shiftsData?.total || 0;
+
+  return (
+    <div className="p-6">
+      <h1 className="text-3xl font-bold mb-6">
+        Welcome to {activeClub?.name || 'WorkClub'}
+      </h1>
+
+      <div className="grid gap-4 md:grid-cols-2 lg:grid-cols-3 mb-6">
+        <Card>
+          <CardHeader>
+            <CardTitle>My Open Tasks</CardTitle>
+          </CardHeader>
+          <CardContent>
+            <div className="text-4xl font-bold">{openTasksCount}</div>
+          </CardContent>
+        </Card>
+
+        <Card>
+          <CardHeader>
+            <CardTitle>My Upcoming Shifts</CardTitle>
+          </CardHeader>
+          <CardContent>
+            <div className="text-4xl font-bold">{upcomingShiftsCount}</div>
+          </CardContent>
+        </Card>
+      </div>
+
+      <div className="flex gap-4">
+        <Link href="/tasks">
+          <Button>View All Tasks</Button>
+        </Link>
+        <Link href="/shifts">
+          <Button variant="outline">View All Shifts</Button>
+        </Link>
+      </div>
+    </div>
+  );
+}