queue method
Queue the email for background sending
Implementation
Future<void> queue() async {
  try {
    validate();
    final content = build();
    final html = content.toHtml();
    final text = content.toText();
    final mail = Mail().toMany(to).subject(subject).html(html).text(text);
    if (cc.isNotEmpty) mail.ccMany(cc);
    if (bcc.isNotEmpty) mail.bccMany(bcc);
    await mail.queue();
    print('📬 Email queued for: ${to.join(', ')}');
  } catch (e) {
    print('❌ Failed to queue email: $e');
    rethrow;
  }
}