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>

frontend/src/lib/types.ts

@@ -159,6 +159,7 @@ export interface SystemSettings {
   smtpPass?: string;
   smtpFrom?: string;
   smtpTls?: boolean;
+  smtpSkipVerify?: boolean;
   smtpConfiguredViaEnv?: boolean;
 }