- Add AppConfig class for dev/prod environment settings
- Create main_dev.dart and main_prod.dart entry points
- Update datasources to use centralized API URL config
- Support --dart-define=ENV for runtime configuration
- Add back button on login page for first-time online mode setup
- Allow users to return to setup screen and change their mode selection
- Add backToSetup localization string for English and German
- Create auth feature with Clean Architecture (domain/data/presentation)
- Add login and register pages with form validation
- Implement secure token storage with flutter_secure_storage
- Create AuthenticatedClient for automatic Bearer token headers
- Add AuthViewModel for global auth state management
- Update router with auth guards (redirect to login if not authenticated)
- Add logout option to settings page
- Update TaskRemoteDataSource to use authenticated client
- Add auth-related localization strings (EN/DE)
