Add validateHostname / validateOptionalHostname helpers to host_utils.dart
that reject values containing @, /, \, or control characters. Wire them
into AddAccountScreen and EditAccountScreen for all host fields.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Hides the SSL/TLS toggle in add/edit account screens when the host is
not localhost; enforces SSL in connectImap/connectSmtp for non-localhost
hosts so plaintext can never be configured accidentally.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>