Using .file kind and .downloading file operation kind will produce a more appropriate localizedAdditionalDescription.
The fileTotalCount and fileCompletedCount aren't appropriate for this case, because there's only one file being downloaded.
I removed this because I thought it wasn't necessary for some reason. We do send a SIGTERM if the user cancels the installation (via subscription cancellation), but if the whole app quits aria2c would keep running in the background.
Like xcodes, storing the username in defaults so we know which item to look up in the keychain later. This also fixes the Xcode list update logic to not only validate the session but login with saved credentials if it fails.