Send a test mail from the admin and log why delivery failed

Form submissions marked as not sent gave no reason: the transport
exception was swallowed. Every send now goes through MailCheck, which
logs the server's answer and a hint in German to var/log/mail.log
(server lines only, no credentials). The system screen shows the mail
route, sends a test mail to any address and lists recent attempts
(ADR 121).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/assets/admin.css

@@ -718,6 +718,11 @@ details.card > summary:hover { background: #fafbf8 }
 .check-bad { color: var(--danger) }
 .log-list { list-style: none; margin: 14px 0 0; padding: 0; display: grid; gap: 6px }
 .log-list code { font-size: 11px; overflow-wrap: anywhere }
+.mail-result { margin-top: 14px }
+.mail-result p { margin: 0 0 6px }
+.mail-result p:last-child { margin-bottom: 0 }
+.mail-result pre { white-space: pre-wrap; overflow-wrap: anywhere; font-size: 11px; margin: 6px 0 0 }
+.mail-log-head { font-size: 14px; margin: 22px 0 0 }
 
 /* ---- the installer: six screens on an otherwise empty server ---------------------------- */