frontend: unify restart-required copy (zh/en)

- Update restartRequired message from model-specific to generic config change
- zh: '切换默认模型后需要重启服务才能生效' -> '配置变更后需要重启服务才能生效'
- en: 'Model changes require a gateway restart' -> 'Configuration changes require a gateway restart'
- Reflects unified restart detection for model/tools/config changes

web/frontend/src/i18n/locales/en.json

@@ -66,7 +66,7 @@
         "restarting": "Restarting Gateway...",
         "stopping": "Stopping Gateway..."
       },
-      "restartRequired": "Model changes require a gateway restart to take effect."
+      "restartRequired": "Configuration changes require a gateway restart to take effect."
     }
   },
   "common": {

web/frontend/src/i18n/locales/zh.json

@@ -66,7 +66,7 @@
         "restarting": "服务重启中...",
         "stopping": "服务停止中..."
       },
-      "restartRequired": "切换默认模型后需要重启服务才能生效。"
+      "restartRequired": "配置变更后需要重启服务才能生效。"
     }
   },
   "common": {