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/DeadlineTrafficLights.tsx

@@ -0,0 +1,105 @@
+"use client";
+
+import { useEffect, useRef } from "react";
+import { AlertTriangle, Clock, CheckCircle } from "lucide-react";
+import type { DeadlineSummary } from "@/lib/types";
+
+function AnimatedCount({ value }: { value: number }) {
+  const ref = useRef<HTMLSpanElement>(null);
+  const prevValue = useRef(value);
+
+  useEffect(() => {
+    const el = ref.current;
+    if (!el || prevValue.current === value) return;
+
+    el.classList.remove("animate-count-up");
+    void el.offsetWidth;
+    el.classList.add("animate-count-up");
+    prevValue.current = value;
+  }, [value]);
+
+  return (
+    <span ref={ref} className="inline-block tabular-nums">
+      {value}
+    </span>
+  );
+}
+
+interface Props {
+  data: DeadlineSummary;
+  onFilter?: (filter: "overdue" | "this_week" | "ok") => void;
+}
+
+export function DeadlineTrafficLights({ data, onFilter }: Props) {
+  const cards = [
+    {
+      key: "overdue" as const,
+      label: "Überfällig",
+      count: data.overdue_count,
+      icon: AlertTriangle,
+      bg: "bg-red-50",
+      border: "border-red-200",
+      iconColor: "text-red-500",
+      countColor: "text-red-700",
+      labelColor: "text-red-600",
+      ring: data.overdue_count > 0 ? "ring-2 ring-red-300 ring-offset-1" : "",
+      pulse: data.overdue_count > 0,
+    },
+    {
+      key: "this_week" as const,
+      label: "Diese Woche",
+      count: data.due_this_week,
+      icon: Clock,
+      bg: "bg-amber-50",
+      border: "border-amber-200",
+      iconColor: "text-amber-500",
+      countColor: "text-amber-700",
+      labelColor: "text-amber-600",
+      ring: "",
+      pulse: false,
+    },
+    {
+      key: "ok" as const,
+      label: "Im Zeitplan",
+      count: data.ok_count + data.due_next_week,
+      icon: CheckCircle,
+      bg: "bg-emerald-50",
+      border: "border-emerald-200",
+      iconColor: "text-emerald-500",
+      countColor: "text-emerald-700",
+      labelColor: "text-emerald-600",
+      ring: "",
+      pulse: false,
+    },
+  ];
+
+  return (
+    <div className="grid grid-cols-1 gap-4 sm:grid-cols-3">
+      {cards.map((card) => (
+        <button
+          key={card.key}
+          onClick={() => onFilter?.(card.key)}
+          className={`group relative overflow-hidden rounded-xl border ${card.border} ${card.bg} ${card.ring} p-6 text-left transition-all hover:shadow-md active:scale-[0.98]`}
+        >
+          {card.pulse && (
+            <span className="absolute right-4 top-4 flex h-3 w-3">
+              <span className="absolute inline-flex h-full w-full animate-ping rounded-full bg-red-400 opacity-75" />
+              <span className="relative inline-flex h-3 w-3 rounded-full bg-red-500" />
+            </span>
+          )}
+          <div className="flex items-center gap-3">
+            <div className={`rounded-lg p-2 ${card.bg}`}>
+              <card.icon className={`h-5 w-5 ${card.iconColor}`} />
+            </div>
+            <span className={`text-sm font-medium ${card.labelColor}`}>
+              {card.label}
+            </span>
+          </div>
+          <div className={`mt-4 text-4xl font-bold tracking-tight ${card.countColor}`}>
+            <AnimatedCount value={card.count} />
+          </div>
+        </button>
+      ))}
+    </div>
+  );
+}