fix(payment): propagate reason/metadata in API error responses

The API client's error interceptor was dropping the reason and metadata
fields from backend error responses. This caused PaymentView to miss
specific error codes (TOO_MANY_PENDING, CANCEL_RATE_LIMITED) and fall
back to generic error messages.

frontend/src/api/client.ts

@@ -92,10 +92,13 @@ apiClient.interceptors.response.use(
         response.data = apiResponse.data
       } else {
         // API error
+        const resp = apiResponse as Record<string, unknown>
         return Promise.reject({
           status: response.status,
           code: apiResponse.code,
-          message: apiResponse.message || 'Unknown error'
+          message: apiResponse.message || 'Unknown error',
+          reason: resp.reason,
+          metadata: resp.metadata,
         })
       }
     }
@@ -268,7 +271,9 @@ apiClient.interceptors.response.use(
         status,
         code: apiData.code,
         error: apiData.error,
-        message: apiData.message || apiData.detail || error.message
+        message: apiData.message || apiData.detail || error.message,
+        reason: apiData.reason,
+        metadata: apiData.metadata,
       })
     }