Fix implicit TLS (SMTPS) handling on SMTP port 465

[RobinMeis]

Fixes #385

Problem

Port 465 is implicit TLS (SMTPS): the server expects the TLS handshake immediately after the TCP connection is opened, with no plaintext greeting and no STARTTLS negotiation. The SMTP client code always configured a TLSPolicy (which only governs STARTTLS), so on port 465 it ended up waiting for a plaintext response the server would never send. The connection would hang or fail instead of succeeding.

Fix

When TLS is enabled and the port is 465, establish TLS immediately instead of relying on STARTTLS:

• internal/service/setup_service.go (TestSMTPConnection): use mail.WithSSL() instead of mail.WithTLSPolicy(...) when UseTLS && Port == 465.
• pkg/mailer/mailer.go (SMTPMailer.createSMTPClient): same change, using mail.WithSSL() for port 465.
• internal/service/smtp_service.go (sendRawEmailWithSettings): wrap the raw connection with tls.Client(conn, tlsConfig) and perform the handshake right after connecting when UseTLS && Port == 465, and skip the subsequent STARTTLS command since the connection is already secured.

All other ports/configurations (STARTTLS on 587, plaintext on 25, etc.) are unaffected. The existing TLSPolicy/STARTTLS behavior is preserved.

Testing

• Verified TestSMTPConnection and outgoing mail sending succeed against a port-465/SMTPS SMTP provider.
• Verified STARTTLS-based providers (e.g. port 587) and unauthenticated/plaintext SMTP still work as before.