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/components/dashboard/AISummaryCard.tsx

@@ -0,0 +1,70 @@
+"use client";
+
+import { Sparkles } from "lucide-react";
+import type { DashboardData } from "@/lib/types";
+
+interface Props {
+  data: DashboardData;
+}
+
+function generateSummary(data: DashboardData): string {
+  const parts: string[] = [];
+  const { deadline_summary: ds, case_summary: cs, upcoming_deadlines: ud } = data;
+
+  // Deadline urgency
+  if (ds.overdue_count > 0) {
+    parts.push(
+      `${ds.overdue_count} Frist${ds.overdue_count > 1 ? "en" : ""} ${ds.overdue_count > 1 ? "sind" : "ist"} überfällig und ${ds.overdue_count > 1 ? "erfordern" : "erfordert"} sofortige Aufmerksamkeit.`,
+    );
+  }
+
+  if (ds.due_this_week > 0) {
+    parts.push(
+      `${ds.due_this_week} Frist${ds.due_this_week > 1 ? "en laufen" : " läuft"} diese Woche ab.`,
+    );
+  }
+
+  // Highlight most critical upcoming deadline
+  if (ud.length > 0) {
+    const next = ud[0];
+    parts.push(
+      `Die nächste Frist ist "${next.title}" in Akte ${next.case_number}.`,
+    );
+  }
+
+  // Case activity
+  if (cs.new_this_month > 0) {
+    parts.push(
+      `${cs.new_this_month} neue Akte${cs.new_this_month > 1 ? "n" : ""} diesen Monat bei ${cs.active_count} aktiven Verfahren.`,
+    );
+  } else {
+    parts.push(`${cs.active_count} aktive Verfahren.`);
+  }
+
+  // All good
+  if (ds.overdue_count === 0 && ds.due_this_week === 0) {
+    parts.unshift("Alle Fristen sind im Zeitplan.");
+  }
+
+  return parts.join(" ");
+}
+
+export function AISummaryCard({ data }: Props) {
+  const summary = generateSummary(data);
+
+  return (
+    <div className="rounded-xl border border-neutral-200 bg-white p-5">
+      <div className="flex items-center gap-2">
+        <div className="rounded-md bg-violet-50 p-1.5">
+          <Sparkles className="h-4 w-4 text-violet-500" />
+        </div>
+        <h2 className="text-sm font-semibold text-neutral-900">
+          KI-Zusammenfassung
+        </h2>
+      </div>
+      <p className="mt-3 text-sm leading-relaxed text-neutral-700">
+        {summary}
+      </p>
+    </div>
+  );
+}