feat: add web gateway hot reload and polling state sync (#1684)

* feat(gateway): support hot reload and empty startup

- extract gateway runtime into pkg/gateway
- add gateway.hot_reload config with default and example values
- allow starting the gateway without a default model via --allow-empty
- stop treating missing enabled channels as a startup error
- update related tests

* feat: replace gateway SSE updates with polling-based state sync

- remove gateway SSE broadcasting and event endpoint
- add polling-based gateway status refresh with stopping state handling
- detect when gateway restart is required after default model changes
- resolve gateway health and websocket proxy targets from configured host
- update gateway UI labels and add backend/frontend test coverage

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

@@ -63,7 +63,8 @@
       },
       "status": {
         "starting": "Starting Gateway...",
-        "restarting": "Restarting Gateway..."
+        "restarting": "Restarting Gateway...",
+        "stopping": "Stopping Gateway..."
       },
       "restartRequired": "Model changes require a gateway restart to take effect."
     }

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

@@ -63,7 +63,8 @@
       },
       "status": {
         "starting": "服务启动中...",
-        "restarting": "服务重启中..."
+        "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。"
     }