New public register.php flow: email + a GD-rendered math CAPTCHA (no
third-party service), a mailed temporary password doubling as email
verification, forced password change on first login. Gated behind a
new ALLOW_SELF_REGISTRATION toggle (default off).
Login moves from username to email (falls back to username for
pre-migration accounts without one yet, mirroring qr-vip's existing
migration 006 pattern) - self-registration needs email as the
identifier. New set_email.php interstitial for legacy accounts.
Adds a small PHPMailer-based Mailer class (SMTP, with an
unauthenticated-relay option via MAIL_SMTP_AUTH=false) since no mail
infrastructure existed in this app before.
