feat: add deadline management frontend (Phase 1G)

- Fristen page with list view (sortable, filterable by status/case)
- Calendar view with month navigation and deadline dots
- Deadline calculator page (proceeding type + trigger date = timeline)
- Traffic light urgency: red (overdue), amber (this week), green (OK)
- Backend: GET /api/deadlines (all tenant deadlines), GET /api/proceeding-types
- API client: added patch() method
- Types: DeadlineRule, ProceedingType, CalculatedDeadline, RuleTreeNode

frontend/src/app/(app)/fristen/rechner/page.tsx

@@ -0,0 +1,26 @@
+"use client";
+
+import { DeadlineCalculator } from "@/components/deadlines/DeadlineCalculator";
+import { ArrowLeft } from "lucide-react";
+import Link from "next/link";
+
+export default function FristenrechnerPage() {
+  return (
+    <div className="space-y-4">
+      <div>
+        <Link
+          href="/fristen"
+          className="mb-2 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 Fristen
+        </Link>
+        <h1 className="text-lg font-semibold text-neutral-900">Fristenrechner</h1>
+        <p className="mt-0.5 text-sm text-neutral-500">
+          Berechnen Sie Fristen basierend auf Verfahrensart und Auslosedatum
+        </p>
+      </div>
+      <DeadlineCalculator />
+    </div>
+  );
+}