feat: add real-time preprocessing preview with side-by-side comparison

- Create PreprocessingPreview component with debounced config updates
- Show original vs preprocessed images side-by-side
- Display image quality metrics (contrast, sharpness) with quality indicators
- Add zoom controls and fullscreen view for detailed inspection
- Show auto-detected configuration when in auto mode
- Integrate preview toggle with PreprocessingSettings component
- Add i18n translations for preview panel UI

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

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

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

@@ -94,7 +94,29 @@
       "advanced": "進階選項",
       "binarize": "二值化處理",
       "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。"
+      "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。",
+      "previewPanel": {
+        "title": "前處理預覽",
+        "loading": "載入預覽中...",
+        "loadError": "載入預覽失敗",
+        "refresh": "重新整理預覽",
+        "original": "原始影像",
+        "preprocessed": "前處理後",
+        "fullscreen": "全螢幕檢視",
+        "qualityAnalysis": "影像品質分析",
+        "contrast": "對比度",
+        "sharpness": "清晰度",
+        "qualityLow": "低",
+        "qualityMedium": "中",
+        "qualityHigh": "高",
+        "qualityBlurry": "模糊",
+        "qualityNormal": "一般",
+        "qualitySharp": "清晰",
+        "autoDetectedConfig": "自動偵測設定",
+        "contrastEnhancement": "對比度增強",
+        "sharpenEnabled": "啟用",
+        "sharpenDisabled": "停用"
+      }
     }
   },
   "results": {