* 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
- add a dedicated /api/gateway/logs endpoint for incremental log polling
- keep /api/gateway/status focused on runtime and health data only
- update frontend log fetching to use the new API and add backend tests covering the status/logs separation and cleared-log behavior
- track boot and config default models in gateway status/events
- preserve running, starting, and restarting states during health checks
- add safer gateway restart handling with stronger backend test coverage
- expose restart-required UI and refresh model state after default model update
