When an Admin user logs in, display a centered dialog allowing them to
select which user identity to use for the session. The Admin adopts the
selected user's permissions and operates as that user throughout the session.
Changes:
- Add UserSelectionDialog class in gui/user_selection_dialog.py
- Add SessionManager.switch_user() method to switch user identity
- Add SessionManager.get_original_admin() method to retrieve original admin
- Update MainWindow status bar to show when Admin operates as another user
- Display format: "当前用户: {username} ({type}) - 以 {admin_username} 身份登录"
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
