feat: add contrast/sharpen strength controls, disable binarization

Major improvements to preprocessing controls:

Backend:
- Add contrast_strength (0.5-3.0) and sharpen_strength (0.5-2.0) to PreprocessingConfig
- Auto-detection now calculates optimal strength based on image quality metrics:
  - Lower contrast → Higher contrast_strength
  - Lower edge strength → Higher sharpen_strength
- Disable binarization in auto mode (rarely beneficial)
- CLAHE clipLimit now scales with contrast_strength
- Sharpening uses unsharp mask with variable strength

Frontend:
- Add strength sliders for contrast and sharpen in manual mode
- Sliders show current value and strength level (輕微/正常/強/最強)
- Move binarize option to collapsible "進階選項" section
- Updated i18n translations for strength labels

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

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

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

@@ -68,9 +68,9 @@
       "title": "影像前處理",
       "mode": {
         "auto": "自動模式",
-        "autoDesc": "系統自動分析影像品質，決定最佳的前處理方式",
+        "autoDesc": "系統自動分析影像品質，決定最佳的前處理方式和強度",
         "manual": "手動模式",
-        "manualDesc": "手動選擇前處理選項，完全控制處理流程",
+        "manualDesc": "手動選擇前處理選項和強度，完全控制處理流程",
         "disabled": "停用前處理",
         "disabledDesc": "不進行任何前處理，直接使用原始影像"
       },
@@ -84,8 +84,16 @@
         "clahe": "CLAHE 自適應均衡化"
       },
       "sharpen": "邊緣銳化",
+      "strength": {
+        "label": "強度",
+        "subtle": "輕微",
+        "normal": "正常",
+        "strong": "強",
+        "maximum": "最強"
+      },
+      "advanced": "進階選項",
       "binarize": "二值化處理",
-      "binarizeWarning": "可能影響顏色資訊",
+      "binarizeWarning": "不建議使用",
       "note": "前處理僅影響版面偵測階段，用於改善表格和文字區塊的識別。原始影像仍用於最終的 OCR 文字提取，確保最佳識別品質。"
     }
   },