Mark implemented tasks as complete and add implementation summary: - Configuration: config.py and schema additions ✓ - Preprocessing service: layout_preprocessing_service.py ✓ - OCR integration: ocr_service.py and pp_structure_enhanced.py ✓ - Preview API endpoints ✓ - Frontend UI: PreprocessingSettings component ✓ - i18n translations (zh-TW) ✓ - Basic unit testing ✓ 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>