Function sign_in_dropbox 

pub async fn sign_in_dropbox(
    key_service: &StoreKeys,
    store_name: &str,
    oauth_cancel: Receiver<bool>,
    clock: &dyn Clock,
) -> Result<String, SetupError>

Dropbox OAuth sign-in: authorize, create the store folder, save tokens to the keyring. Returns the folder path for the host to persist in its config.

Uses the localhost-callback flow; gated on oauth-providers.