feat: add preprocessing UI components and integration

Frontend implementation for add-layout-preprocessing proposal:
- Add PreprocessingSettings component with mode selection (auto/manual/disabled)
- Add manual config panel (contrast, sharpen, binarize options)
- Add zh-TW translations for preprocessing UI
- Integrate with ProcessingPage task start flow
- Add preprocessing types to apiV2.ts (PreprocessingMode, PreprocessingConfig)
- Pass preprocessing options to task start API

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

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

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

@@ -63,6 +63,30 @@
       "cdlaDesc": "CDLA 版面分析模型 (~86% mAP) - 專用中文版面分析",
       "recommended": "推薦",
       "note": "版面模型會影響文件結構（表格、文字區塊、圖片）的偵測效果。請根據您的文件類型選擇適合的模型。"
+    },
+    "preprocessing": {
+      "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",
+        "manual": "手動模式",
+        "manualDesc": "手動選擇前處理選項，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"
+      },
+      "recommended": "推薦",
+      "preview": "預覽效果",
+      "manualConfig": "手動設定選項",
+      "contrast": {
+        "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"
+      },
+      "sharpen": "邊緣銳化",
+      "binarize": "二值化處理",
+      "binarizeWarning": "可能影響顏色資訊",
+      "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。"
     }
   },
   "results": {