feat: add frontend dashboard with traffic lights, timeline, and AI summary (Phase 2L)

Dashboard page at /dashboard with 5 components:
- DeadlineTrafficLights: RED/AMBER/GREEN cards with animated counts and pulse for overdue
- CaseOverviewGrid: active/new/closed case counts
- UpcomingTimeline: merged deadlines + appointments for next 7 days, grouped by day
- AISummaryCard: natural language summary generated from dashboard data
- QuickActions: shortcuts to create cases, deadlines, AI analysis, CalDAV sync

3-column responsive grid layout. Root / redirects to /dashboard.
Fetches from GET /api/dashboard with 60s auto-refresh via react-query.

frontend/src/app/(app)/page.tsx

@@ -1,10 +1,5 @@
-export default function DashboardPage() {
-  return (
-    <div>
-      <h1 className="text-lg font-semibold text-neutral-900">Dashboard</h1>
-      <p className="mt-1 text-sm text-neutral-500">
-        Willkommen bei KanzlAI
-      </p>
-    </div>
-  );
+import { redirect } from "next/navigation";
+
+export default function RootPage() {
+  redirect("/dashboard");
 }