MailKit/MimeKit are unnecessary third-party dependencies with known
vulnerabilities. .NET's built-in System.Net.Mail.SmtpClient handles
the same test send without any additional packages.
Ref: IT-628
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
- POST /admin/channels/{id}/test — direct SMTP send via MailKit
- Test button shown only on existing channels (not create)
- Bootstrap modal with recipient email input and spinner
- Inline success/error result inside the modal
Ref: IT-628
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>