feat: implement layout-preserving PDF generation with table reconstruction

Major Features:
- Add PDF generation service with Chinese font support
- Parse HTML tables from PP-StructureV3 and rebuild with ReportLab
- Extract table text for translation purposes
- Auto-filter text regions inside tables to avoid overlaps

Backend Changes:
1. pdf_generator_service.py (NEW)
   - HTMLTableParser: Parse HTML tables to extract structure
   - PDFGeneratorService: Generate layout-preserving PDFs
   - Coordinate transformation: OCR (top-left) → PDF (bottom-left)
   - Font size heuristics: 75% of bbox height with width checking
   - Table reconstruction: Parse HTML → ReportLab Table
   - Image embedding: Extract bbox from filenames

2. ocr_service.py
   - Add _extract_table_text() for translation support
   - Add output_dir parameter to save images to result directory
   - Extract bbox from image filenames (img_in_table_box_x1_y1_x2_y2.jpg)

3. tasks.py
   - Update process_task_ocr to use save_results() with PDF generation
   - Fix download_pdf endpoint to use database-stored PDF paths
   - Support on-demand PDF generation from JSON

4. config.py
   - Add chinese_font_path configuration
   - Add pdf_enable_bbox_debug flag

Frontend Changes:
1. PDFViewer.tsx (NEW)
   - React PDF viewer with zoom and pagination
   - Memoized file config to prevent unnecessary reloads

2. TaskDetailPage.tsx & ResultsPage.tsx
   - Integrate PDF preview and download

3. main.tsx
   - Configure PDF.js worker via CDN

4. vite.config.ts
   - Add host: '0.0.0.0' for network access
   - Use VITE_API_URL environment variable for backend proxy

Dependencies:
- reportlab: PDF generation library
- Noto Sans SC font: Chinese character support

🤖 Generated with Claude Code
https://claude.com/claude-code

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

requirements.txt

@@ -21,6 +21,7 @@ opencv-python>=4.8.0
 # ===== PDF Generation =====
 weasyprint>=60.0
 markdown>=3.5.0
+reportlab>=4.0.0  # Layout-preserving PDF generation with precise coordinate control
 # Note: pandoc needs to be installed via brew (brew install pandoc)
 
 # ===== Data Export =====