Don't make mail required because the api credentials are enought.

[Nemirtingas]

You don't need to pass in the email because its in the Profile struct :

syncData = cli.sync()
syncData.Profile.Email

[Lujeni]

Good first issue

This should be a straightforward change, email is only used as the salt for master key derivation, and it's already available from sync().Profile.Email after authN. So we can make it optional and lazily resolve it from the API when not provided.

The auth itself (client_id + client_secret) doesn't need the email at all, so there's no circular dependency issue.