Catch Email Errors on signup

Feature Request

When signing up without having a correct email configuration at the moment, the modal fails and shows "oops, something went wrong" but the user is still created. This leads to a poor UX.
Instead, the error should be caught, and the email logged - perhaps if we set the default mail driver to "log" on flarum creation also?

Describe the solution you'd like
A detailed description of your proposed solution. Include:

• Default Flarum installations should set mail driver to log.
• Wrap the signup in a try/catch and catch the failed email as a log.
• User should be suspended pending activation, as the user has still been created - just no email sent.

Justify why this feature belongs in Flarum's core, rather than in a third-party extension
Core feature - Signup.

REQUIRES DISCUSSION

Start by locating the signup flow and the mail configuration entry point; the issue names no files or tests. Resolve whether the default mail driver, failed-email logging, and suspended-pending-activation behavior belong in the proposed scope, then verify that signup no longer shows an error after creating a user without sending mail.