When switching identities, the managesieve plugin still connected to
the default sieve server. Hook into managesieve_connect to redirect
the sieve connection to the remote account's server.
Adds sieve_host, sieve_port and sieve_auth columns to the database,
a Sieve section to the identity settings form, preconfig support for
sieve_host, and localized labels for all 7 languages.
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.
