- Account model gains `username` field (default empty → falls back to email then local-part)
- ConnectionTestService returns the effective username that succeeded; tries email then local-part when blank
- JMAP connection probe uses Basic-auth GET to /.well-known/jmap (401/403 = auth failure)
- IMAP/SMTP factory passes explicit username parameter
- Add/edit account screens show username field and "Try connection" button
- EditAccountScreen reuses stored password when no new password is entered
- Unit tests for ConnectionTestServiceImpl (IMAP + JMAP paths, fallback logic)
- Fix unit test lambda signatures for updated ImapConnectFn/SmtpConnectFn
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
