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/backend/api/router.go

@@ -71,7 +71,4 @@ func (h *Handler) RegisterRoutes(mux *http.ServeMux) {
 	h.registerLauncherConfigRoutes(mux)
 }
 
-// Shutdown gracefully shuts down the handler, closing all SSE connections.
-func (h *Handler) Shutdown() {
-	gateway.events.Shutdown()
-}
+func (h *Handler) Shutdown() {}