fix: harden online validation flows

frontend/src/lib/api.ts

@@ -55,7 +55,20 @@ async function request<T>(
   Object.assign(headers, options.headers as Record<string, string>);
   if (token) headers['Authorization'] = `Bearer ${token}`;
 
-  const res = await fetch(`${BASE_URL}${path}`, { ...options, headers });
+  const url = `${BASE_URL}${path}`;
+  const res = await fetch(url, { ...options, headers });
+  const contentType = res.headers.get('content-type') || '';
+  if (!contentType.includes('application/json')) {
+    const text = await res.text();
+    const apiError = new Error(
+      `接口返回非 JSON(${res.status})，请刷新后重试；如果仍出现，请联系管理员检查后端代理。`
+    ) as Error & { code?: number; status?: number; responseText?: string; url?: string };
+    apiError.code = res.status || 50001;
+    apiError.status = res.status;
+    apiError.responseText = text.slice(0, 160);
+    apiError.url = url;
+    throw apiError;
+  }
   const body: ApiResponse<T> | ApiError = await res.json();
 
   if ('code' in body && body.code === ERROR_CODES.SUCCESS) {