This PR enhances the version checker to send a custom User-Agent header when polling for updates, and configures both the management CLI and client UI to use distinct agents.
- NewUpdate now takes an `httpAgent` string to set the User-Agent header.
- `fetchVersion` builds a custom HTTP request (instead of `http.Get`) and sets the User-Agent.
- Management CLI and client UI now pass `"nb/management"` and `"nb/client-ui"` respectively to NewUpdate.
- Tests updated to supply an `httpAgent` constant.
- Logs if there is a new version available for management
Periodically fetch the latest available version, and the UI will shows a new menu for the download link. It checks both the daemon version and the UI version.
