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

config/config.example.json

@@ -518,6 +518,7 @@
   },
   "gateway": {
     "host": "127.0.0.1",
-    "port": 18790
+    "port": 18790,
+    "hot_reload": false
   }
 }