fix: handle empty 200 responses in adminFetch to fix stale UI after mutations

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/api/queries/admin/admin-api.ts

@@ -18,5 +18,7 @@ export async function adminFetch<T>(path: string, options?: RequestInit): Promis
   }
   if (!res.ok) throw new Error(`API error: ${res.status}`);
   if (res.status === 204) return undefined as T;
-  return res.json();
+  const text = await res.text();
+  if (!text) return undefined as T;
+  return JSON.parse(text);
 }