Add SecureStore integration to ConfigManager for secure handling of connection
strings and sensitive configuration values. Includes store/secret management
UI, encrypted .store file support, and comprehensive test coverage.
- Add IFileSystem and IDialogService dependencies to constructor
- Implement OnSelectedNodeChanged to create appropriate form ViewModels
- Add LoadConfigForTesting helper for unit testing
- Add MarkAsChanged helper to track unsaved changes
- Update OpenFolderAsync to use IDialogService
- Add comprehensive unit tests for form selection