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.
Add proper copyright notices to all source files with accurate date
ranges per contributor based on git history. Add contributors to
composer.json authors and README.
