fix: extract actual error message from backend error responses

The error interceptor was re-throwing the generic axios Error object
instead of extracting the actual error message from the response body.
Now extracts error.response.data.error for meaningful error messages.

frontend/src/api/index.js

@@ -39,7 +39,10 @@ service.interceptors.response.use(
     return res
   },
   error => {
-    console.error('Response error:', error)
+    // Extract actual error message from response body
+    const backendError = error.response?.data?.error || error.response?.data?.message
+    const errorMsg = backendError || error.message || 'Unknown error'
+    console.error('Response error:', errorMsg, error)
     
     // 处理超时
     if (error.code === 'ECONNABORTED' && error.message.includes('timeout')) {
@@ -51,7 +54,10 @@ service.interceptors.response.use(
       console.error('Network error - please check your connection')
     }
     
-    return Promise.reject(error)
+    // Reject with a meaningful error message
+    const enhancedError = new Error(errorMsg)
+    enhancedError.response = error.response
+    return Promise.reject(enhancedError)
   }
 )