The preconfig previously used a single 'host' field for both IMAP and
SMTP, making it impossible to configure different servers/schemes/ports
(e.g. IMAPS on 993 + SMTP STARTTLS on 587). Also, ssl:// scheme was
silently lost during parse_url() since only tls:// was recognized.
Add imap_host and smtp_host as separate config keys with full scheme
support (ssl://, tls://). The legacy 'host' key is still supported as
fallback for backward compatibility.
9ebe30c40a