Improve UX when both, app and file backup are disabled

Source of this issue: https://github.com/seedvault-app/seedvault/discussions/1033

Desired solution

In case when user attempts to create first backup, without selected any options (no things to backup) the app should either notify about situation There is nothing to backup/No resources to backup selected or it should be not possible in the UI to enable and schedule backups.

Current state

Right now after triggering backup nothing happens. This may be misleading for users (for example user may just enable scheduled backups and just expect it to work with defaults).