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)/dashboard/page.tsx

@@ -0,0 +1,69 @@
+"use client";
+
+import { useQuery } from "@tanstack/react-query";
+import { api } from "@/lib/api";
+import type { DashboardData } from "@/lib/types";
+import { DeadlineTrafficLights } from "@/components/dashboard/DeadlineTrafficLights";
+import { CaseOverviewGrid } from "@/components/dashboard/CaseOverviewGrid";
+import { UpcomingTimeline } from "@/components/dashboard/UpcomingTimeline";
+import { AISummaryCard } from "@/components/dashboard/AISummaryCard";
+import { QuickActions } from "@/components/dashboard/QuickActions";
+import { Loader2 } from "lucide-react";
+
+export default function DashboardPage() {
+  const { data, isLoading, error } = useQuery({
+    queryKey: ["dashboard"],
+    queryFn: () => api.get<DashboardData>("/dashboard"),
+    refetchInterval: 60_000,
+  });
+
+  if (isLoading) {
+    return (
+      <div className="flex h-full items-center justify-center">
+        <Loader2 className="h-6 w-6 animate-spin text-neutral-400" />
+      </div>
+    );
+  }
+
+  if (error || !data) {
+    return (
+      <div className="flex h-full items-center justify-center">
+        <p className="text-sm text-neutral-500">
+          Dashboard konnte nicht geladen werden.
+        </p>
+      </div>
+    );
+  }
+
+  return (
+    <div className="mx-auto max-w-6xl space-y-6">
+      <div>
+        <h1 className="text-lg font-semibold text-neutral-900">Dashboard</h1>
+        <p className="mt-0.5 text-sm text-neutral-500">
+          Fristenübersicht und Kanzlei-Status
+        </p>
+      </div>
+
+      {/* Traffic Lights — the hero section */}
+      <DeadlineTrafficLights data={data.deadline_summary} />
+
+      {/* Main content grid */}
+      <div className="grid grid-cols-1 gap-6 lg:grid-cols-3">
+        {/* Left column: Timeline (takes 2 cols) */}
+        <div className="lg:col-span-2">
+          <UpcomingTimeline
+            deadlines={data.upcoming_deadlines}
+            appointments={data.upcoming_appointments}
+          />
+        </div>
+
+        {/* Right column: Case overview, AI summary, Quick actions */}
+        <div className="space-y-6">
+          <CaseOverviewGrid data={data.case_summary} />
+          <AISummaryCard data={data} />
+          <QuickActions />
+        </div>
+      </div>
+    </div>
+  );
+}