- Implement MailRootView with sidebar, thread list, and detail views.
- Create MailSidebarView for mailbox navigation and filters.
- Add ThreadListView and ThreadDetailView for displaying threads and messages.
- Introduce ComposeView for composing new emails.
- Implement adaptive UI for different platforms (iOS/macOS).
- Add MailTheme for consistent styling across components.
- Create unit tests for AppNavigationCommand and AppViewModel to ensure functionality.
