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.
Extract business logic into dedicated classes under lib/:
- IdentSwitchForm: form building, validation, and persistence
- IdentSwitchSwitcher: account switching, SMTP config, special folders
- IdentSwitchPreconfig: domain-based preconfiguration
The main plugin class remains the orchestrator, registering hooks
and delegating to the appropriate handler. Constants changed from
private to public to allow cross-class access.
