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/services/apiV2.ts

@@ -33,6 +33,8 @@ import type {
   ProcessingOptions,
   ProcessingMetadata,
   DocumentAnalysisResponse,
+  PreprocessingPreviewRequest,
+  PreprocessingPreviewResponse,
 } from '@/types/apiV2'
 
 /**
@@ -517,6 +519,41 @@ class ApiClientV2 {
     window.URL.revokeObjectURL(link.href)
   }
 
+  // ==================== Preprocessing Preview APIs ====================
+
+  /**
+   * Preview preprocessing effect on a page
+   */
+  async previewPreprocessing(
+    taskId: string,
+    request: PreprocessingPreviewRequest
+  ): Promise<PreprocessingPreviewResponse> {
+    const response = await this.client.post<PreprocessingPreviewResponse>(
+      `/tasks/${taskId}/preview/preprocessing`,
+      request
+    )
+    return response.data
+  }
+
+  /**
+   * Get preprocessing preview image URL (with auth token)
+   */
+  getPreviewImageUrl(taskId: string, type: 'original' | 'preprocessed', page: number): string {
+    const baseUrl = `${API_BASE_URL}/api/${API_VERSION}/tasks/${taskId}/preview/image`
+    return `${baseUrl}?type=${type}&page=${page}`
+  }
+
+  /**
+   * Get preview image as blob (with authentication)
+   */
+  async getPreviewImage(taskId: string, type: 'original' | 'preprocessed', page: number): Promise<Blob> {
+    const response = await this.client.get(`/tasks/${taskId}/preview/image`, {
+      params: { type, page },
+      responseType: 'blob',
+    })
+    return response.data
+  }
+
   // ==================== Admin APIs ====================
 
   /**