Allow custom email domain definition to support app subdomain using the main domain

Currently - when using a custom subdomain, the custom email setup can be defined with the subdomain but not the main domain - e.g. if the domain is mysite.com, and the app sits in support.mysite.com the email can be defined to hello@support.mysite.com

This is a problem, as the main domain may be needed for external website or another base44 app using the main domain.

Custom email definition should allow (or even default) to an email within the main domain. Currently not possible.

That is, if the base44 app sits at support.mysite.com it should be possible to define the custom email as hello@mysite.com