fix: bulk edit mixed channel warning not showing confirmation dialog

The response interceptor in client.ts transforms errors into plain
objects {status, code, message}, but catch blocks were checking
error.response?.status (AxiosError format) which never matched.

- Add error field passthrough in client.ts interceptor
- Refactor BulkEditAccountModal to use pre-check API (checkMixedChannelRisk)
  before submit, matching the single edit flow
- Fix EditAccountModal catch blocks to use interceptor error format
- Add bulk-update mixed channel unit tests

frontend/src/api/client.ts

@@ -267,6 +267,7 @@ apiClient.interceptors.response.use(
       return Promise.reject({
         status,
         code: apiData.code,
+        error: apiData.error,
         message: apiData.message || apiData.detail || error.message
       })
     }