When login with a social account, we check whether this social account is linked with an existing Vijos account, if not (first time), prompt for:
Link to an existing Vijos account (login required), or
Create a new Vijos account (only need to input the nickname), after creation this Vijos account is unverified, username is system-assigned and password is unspecified (cannot be logged in via password unless user changes password).
When login with a social account, we check whether this social account is linked with an existing Vijos account, if not (first time), prompt for: