Function sign_in_google_drive 

pub async fn sign_in_google_drive(
    key_service: &KeyService,
    library_name: &str,
    oauth_cancel: Receiver<bool>,
    clock: &dyn Clock,
) -> Result<String, SetupError>

Google Drive OAuth sign-in: authorize, find/create the library folder, save tokens to the keyring. Returns the folder id for the host to persist in its own config (coven never writes the host’s config).