Implement deferred code review findings:
- Add IDialogService/IClipboardService interfaces for testable platform operations
- Create AvaloniaDialogService and AvaloniaClipboardService implementations
- Extract dialog strings and file extensions to centralized Constants classes
- Refactor ViewModels to use DI instead of event delegates
- Update tests to use mock services
- Fix CriteriaSheetGenerator.FormatTimestamp to handle all DateTimeKind values
- Update TestWebApplicationFactory to use IAuthenticationService
- Add logger parameter to ExcelParserServiceTests
- Add SecureStoreManager to solution under /utils/ folder