- 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)
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