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]/fristen/page.tsx

@@ -0,0 +1,86 @@
+"use client";
+
+import { useQuery } from "@tanstack/react-query";
+import { useParams } from "next/navigation";
+import { api } from "@/lib/api";
+import type { Deadline } from "@/lib/types";
+import { format } from "date-fns";
+import { de } from "date-fns/locale";
+import { Clock, Loader2 } from "lucide-react";
+
+const DEADLINE_STATUS: Record<string, string> = {
+  pending: "bg-amber-50 text-amber-700",
+  completed: "bg-emerald-50 text-emerald-700",
+  overdue: "bg-red-50 text-red-700",
+};
+
+const DEADLINE_STATUS_LABEL: Record<string, string> = {
+  pending: "Offen",
+  completed: "Erledigt",
+  overdue: "Ueberfaellig",
+};
+
+export default function FristenPage() {
+  const { id } = useParams<{ id: string }>();
+
+  const { data, isLoading } = useQuery({
+    queryKey: ["case-deadlines", id],
+    queryFn: () =>
+      api.get<{ deadlines: Deadline[]; total: number }>(
+        `/deadlines?case_id=${id}`,
+      ),
+  });
+
+  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 deadlines = Array.isArray(data?.deadlines) ? data.deadlines : [];
+
+  if (deadlines.length === 0) {
+    return (
+      <div className="flex flex-col items-center py-8 text-center">
+        <div className="rounded-xl bg-neutral-100 p-3">
+          <Clock className="h-5 w-5 text-neutral-400" />
+        </div>
+        <p className="mt-2 text-sm text-neutral-500">
+          Keine Fristen vorhanden.
+        </p>
+      </div>
+    );
+  }
+
+  return (
+    <div className="space-y-2">
+      {deadlines.map((d) => (
+        <div
+          key={d.id}
+          className="flex flex-col gap-2 rounded-md border border-neutral-200 bg-white px-4 py-3 sm:flex-row sm:items-center sm:justify-between"
+        >
+          <div>
+            <p className="text-sm font-medium text-neutral-900">{d.title}</p>
+            {d.description && (
+              <p className="mt-0.5 text-sm text-neutral-500">
+                {d.description}
+              </p>
+            )}
+          </div>
+          <div className="flex items-center gap-3">
+            <span
+              className={`rounded-full px-2 py-0.5 text-xs font-medium ${DEADLINE_STATUS[d.status] ?? "bg-neutral-100 text-neutral-500"}`}
+            >
+              {DEADLINE_STATUS_LABEL[d.status] ?? d.status}
+            </span>
+            <span className="whitespace-nowrap text-sm text-neutral-500">
+              {format(new Date(d.due_date), "d. MMM yyyy", { locale: de })}
+            </span>
+          </div>
+        </div>
+      ))}
+    </div>
+  );
+}