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/CaseOverviewGrid.tsx

@@ -0,0 +1,55 @@
+"use client";
+
+import { FolderOpen, FolderPlus, Archive } from "lucide-react";
+import type { CaseSummary } from "@/lib/types";
+
+interface Props {
+  data: CaseSummary;
+}
+
+export function CaseOverviewGrid({ data }: Props) {
+  const items = [
+    {
+      label: "Aktive Akten",
+      value: data.active_count,
+      icon: FolderOpen,
+      color: "text-blue-600",
+      bg: "bg-blue-50",
+    },
+    {
+      label: "Neu (Monat)",
+      value: data.new_this_month,
+      icon: FolderPlus,
+      color: "text-violet-600",
+      bg: "bg-violet-50",
+    },
+    {
+      label: "Abgeschlossen",
+      value: data.closed_count,
+      icon: Archive,
+      color: "text-neutral-500",
+      bg: "bg-neutral-50",
+    },
+  ];
+
+  return (
+    <div className="rounded-xl border border-neutral-200 bg-white p-5">
+      <h2 className="text-sm font-semibold text-neutral-900">Aktenübersicht</h2>
+      <div className="mt-4 space-y-3">
+        {items.map((item) => (
+          <div key={item.label} className="flex items-center justify-between">
+            <div className="flex items-center gap-2.5">
+              <div className={`rounded-md p-1.5 ${item.bg}`}>
+                <item.icon className={`h-4 w-4 ${item.color}`} />
+              </div>
+              <span className="text-sm text-neutral-600">{item.label}</span>
+            </div>
+            <span className="text-lg font-semibold tabular-nums text-neutral-900">
+              {item.value}
+            </span>
+          </div>
+        ))}
+      </div>
+    </div>
+  );
+}