feat: add frontend case list, detail, and creation pages (Phase 1F)

- Case list page (/cases) with search, status/type filters, status badges
- Case creation page (/cases/new) with reusable CaseForm component
- Case detail page (/cases/[id]) with tabs: Timeline, Deadlines, Documents, Parties
- CaseTimeline component for chronological case_events display
- PartyList component with inline party CRUD (add/delete)
- Updated sidebar navigation to route to /cases

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

@@ -0,0 +1,49 @@
+"use client";
+
+import { useMutation, useQueryClient } from "@tanstack/react-query";
+import { useRouter } from "next/navigation";
+import { toast } from "sonner";
+import { api } from "@/lib/api";
+import type { Case } from "@/lib/types";
+import { CaseForm, type CaseFormData } from "@/components/cases/CaseForm";
+import { ArrowLeft } from "lucide-react";
+import Link from "next/link";
+
+export default function NewCasePage() {
+  const router = useRouter();
+  const queryClient = useQueryClient();
+
+  const mutation = useMutation({
+    mutationFn: (data: CaseFormData) => api.post<Case>("/cases", data),
+    onSuccess: (created) => {
+      queryClient.invalidateQueries({ queryKey: ["cases"] });
+      toast.success("Akte angelegt");
+      router.push(`/cases/${created.id}`);
+    },
+    onError: () => {
+      toast.error("Fehler beim Anlegen der Akte");
+    },
+  });
+
+  return (
+    <div className="mx-auto max-w-2xl">
+      <Link
+        href="/cases"
+        className="mb-4 inline-flex items-center gap-1 text-sm text-neutral-500 hover:text-neutral-700"
+      >
+        <ArrowLeft className="h-3.5 w-3.5" />
+        Zuruck zu Akten
+      </Link>
+      <h1 className="text-lg font-semibold text-neutral-900">Neue Akte</h1>
+      <p className="mt-0.5 text-sm text-neutral-500">
+        Neue Akte im System anlegen
+      </p>
+      <div className="mt-6 rounded-md border border-neutral-200 bg-white p-6">
+        <CaseForm
+          onSubmit={(data) => mutation.mutate(data)}
+          isSubmitting={mutation.isPending}
+        />
+      </div>
+    </div>
+  );
+}