feat: refactor case detail from useState tabs to URL-based nested routes

Refactors the monolithic cases/[id]/page.tsx into Next.js nested routes
with a shared layout for the case header and tab navigation bar.

Route structure:
- cases/[id]/layout.tsx — case header + tab bar (active tab from URL)
- cases/[id]/page.tsx — redirects to ./verlauf
- cases/[id]/verlauf/page.tsx — timeline tab
- cases/[id]/fristen/page.tsx — deadlines tab
- cases/[id]/dokumente/page.tsx — documents tab (with upload)
- cases/[id]/parteien/page.tsx — parties tab
- cases/[id]/notizen/page.tsx — notes tab (new, uses NotesList)

New shared components:
- Breadcrumb.tsx — reusable breadcrumb navigation
- NotesList.tsx — reusable notes CRUD (inline create/edit/delete)
- Note type added to types.ts

Benefits: deep linking, browser back/forward, bookmarkable tabs.

frontend/src/app/(app)/cases/[id]/dokumente/page.tsx

@@ -0,0 +1,35 @@
+"use client";
+
+import { useQuery } from "@tanstack/react-query";
+import { useParams } from "next/navigation";
+import { api } from "@/lib/api";
+import type { Document } from "@/lib/types";
+import { DocumentList } from "@/components/documents/DocumentList";
+import { DocumentUpload } from "@/components/documents/DocumentUpload";
+import { Loader2 } from "lucide-react";
+
+export default function DokumentePage() {
+  const { id } = useParams<{ id: string }>();
+
+  const { data, isLoading } = useQuery({
+    queryKey: ["case-documents", id],
+    queryFn: () => api.get<Document[]>(`/cases/${id}/documents`),
+  });
+
+  if (isLoading) {
+    return (
+      <div className="flex items-center justify-center py-8">
+        <Loader2 className="h-5 w-5 animate-spin text-neutral-400" />
+      </div>
+    );
+  }
+
+  const documents = Array.isArray(data) ? data : [];
+
+  return (
+    <div className="space-y-6">
+      <DocumentUpload caseId={id} />
+      <DocumentList documents={documents} caseId={id} />
+    </div>
+  );
+}