SmtpRelay: replace mail() stub with native socket SMTP client

TODO.md

@@ -5,4 +5,5 @@
 - [x] Replace JS toast system with pure HTMX toast fragment (top-right, CSS-only auto-fade)
 - [x] Separate admin views from controllers: move all HTML to `templates/admin/*.php`, fragments to `templates/admin/partials/`
 - [x] Add SMTP test email button in parametres.php (action + CSS)
+- [x] Fix SmtpRelay::send() — replace broken mail() stub with native PHP socket SMTP client (STARTTLS/SSL, AUTH PLAIN/LOGIN)
 - [x] Lock body scroll on all pages (admin + public); only `main` or inner element scrolls