Mail sent to someone who is not in the sender's world (system mail with
no address, and all player-to-player mail) now reaches them: the world
sends a MailNotify (Chat::MAIL, unused until now) to the chat server,
which passes it to the world the receiver is in, and that world sends
the client its unread count with a NewMail NotificationResponse.
Receivers in the same world are told directly. Player-to-player mail
did not notify the receiver at all before.
Replaces the TODO in Mail::SendMail.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
d43cdef104