feat: Add Instance Details ViewModel and UI for managing instance credentials
- Introduced InstanceDetailsViewModel to handle loading and displaying instance-specific credentials. - Created InstanceDetailsWindow and associated XAML for displaying admin, database, and OAuth2 credentials. - Updated InstancesViewModel to include command for opening instance details. - Enhanced SettingsViewModel to manage Bitwarden and Xibo Bootstrap configurations, including connection testing. - Added UI components for Bitwarden Secrets Manager and Xibo Bootstrap OAuth2 settings in the SettingsView. - Implemented password visibility toggles and clipboard copy functionality for sensitive information.
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