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]/notizen/page.tsx

@@ -0,0 +1,10 @@
+"use client";
+
+import { useParams } from "next/navigation";
+import { NotesList } from "@/components/notes/NotesList";
+
+export default function NotizenPage() {
+  const { id } = useParams<{ id: string }>();
+
+  return <NotesList parentType="case" parentId={id} />;
+}