Function check_for_update 

pub async fn check_for_update(
    app: AppHandle,
    scheduler: State<'_, Scheduler>,
) -> Result<UpdateInfo, String>

Ask tauri-plugin-updater whether a newer build is available.

Delegates to app.updater()?.check(), which fetches the signed manifest from plugins.updater.endpoints (configured in tauri.conf.json), verifies its signature against the bundled plugins.updater.pubkey, and compares versions with the plugin’s SemVer default. Errors stringify the underlying plugin / transport failure for display in the About tab.