Fix incorrect body data handling

apps/frontend/app/lib/api.ts

@@ -65,7 +65,34 @@ function axiosResponseToFetchResponse(response: AxiosResponse): Response {
     }
   });
 
-  return new Response(response.data, {
+  // Normalize Axios response.data to a Fetch-compatible BodyInit
+  let body: BodyInit | null = null;
+  const data = response.data;
+
+  if (data == null) {
+    body = null;
+  } else if (
+    typeof data === 'string' ||
+    data instanceof Blob ||
+    data instanceof ArrayBuffer ||
+    ArrayBuffer.isView(data) ||
+    data instanceof FormData ||
+    data instanceof URLSearchParams
+  ) {
+    body = data as BodyInit;
+  } else {
+    try {
+      body = JSON.stringify(data);
+      if (!headers.has('content-type')) {
+        headers.set('content-type', 'application/json;charset=utf-8');
+      }
+    } catch {
+      console.warn('Failed to stringify response data as JSON, falling back to string conversion.');
+      body = String(data);
+    }
+  }
+
+  return new Response(body, {
     status: response.status,
     statusText: response.statusText,
     headers: headers,