feat: enhance layout preprocessing and unify image scaling proposal

Backend changes:
- Add image scaling configuration for PP-Structure processing
- Enhance layout preprocessing service with scaling support
- Update OCR service with improved memory management
- Add PP-Structure enhanced processing improvements

Frontend changes:
- Update preprocessing settings UI
- Fix processing page layout and state management
- Update API types for new parameters

Proposals:
- Archive add-layout-preprocessing proposal (completed)
- Add unify-image-scaling proposal for consistent coordinate handling

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/i18n/locales/zh-TW.json

@@ -81,7 +81,8 @@
         "label": "對比度增強",
         "none": "不增強",
         "histogram": "直方圖均衡化",
-        "clahe": "CLAHE 自適應均衡化"
+        "clahe": "CLAHE 自適應均衡化",
+        "document": "掃描件優化 (背景校正+CLAHE)"
       },
       "sharpen": "邊緣銳化",
       "strength": {