Function transfer_download_many 

pub async fn transfer_download_many(
    specs: Vec<TransferDownloadSpec>,
    queue: State<'_, TransferQueueHandle>,
    pool: State<'_, S3ClientPoolHandle>,
    locks: State<'_, LockRegistryHandle>,
    store: State<'_, ProfileStoreHandle>,
    log: State<'_, NotificationLogHandle>,
    settings: State<'_, SettingsHandle>,
    channel: AppHandle,
) -> Result<Vec<String>, AppError>

Bulk-enqueue multiple downloads.

Returns a Vec<String> of request IDs in the same order as specs.