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/lib/api.ts

@@ -69,6 +69,13 @@ class ApiClient {
     });
   }
 
+  patch<T>(path: string, body?: unknown) {
+    return this.request<T>(path, {
+      method: "PATCH",
+      body: body ? JSON.stringify(body) : undefined,
+    });
+  }
+
   delete<T>(path: string) {
     return this.request<T>(path, { method: "DELETE" });
   }