fix(smtp): proper SMTP dialogue, encoded mails, optional skip-verify

- Explicit SMTP flow (dial -> EHLO -> STARTTLS -> AUTH -> MAIL/RCPT/DATA)
  with errors naming the refusing step (also logged as smtp_step)
- EHLO with the sender's domain instead of "localhost"
- Envelope gets the bare address; "Name <addr>" stays in the From header
  (was rejected as invalid MAIL FROM)
- Date and Message-ID headers, RFC 2047 subject, quoted-printable body
- SMTP_SKIP_VERIFY / admin switch for self-signed certificates;
  SMTP checkboxes are switches now
- Tests against a fake SMTP server (STARTTLS, 451, display names)

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

cmd/server/main.go

@@ -38,7 +38,7 @@ func main() {
 	defer store.Close()
 
 	// Seed SMTP defaults from environment variables into DB if not yet configured in DB
-	store.SeedSMTPDefaultsFromEnv(cfg.SMTPHost, cfg.SMTPPort, cfg.SMTPUser, cfg.SMTPPass, cfg.SMTPFrom, cfg.SMTPTLS)
+	store.SeedSMTPDefaultsFromEnv(cfg.SMTPHost, cfg.SMTPPort, cfg.SMTPUser, cfg.SMTPPass, cfg.SMTPFrom, cfg.SMTPTLS, cfg.SMTPSkipVerify)
 
 	if cfg.DemoMode {
 		slog.Info("🧪 Demo-Modus aktiv (MINDA_DEMO=true): Richte Demo-Benutzer admin/admin und user/user ein...")