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>

backend/test_chinese_font.py

@@ -0,0 +1,62 @@
+"""
+Test script to verify ReportLab and Chinese font rendering
+"""
+from reportlab.pdfgen import canvas
+from reportlab.pdfbase import pdfmetrics
+from reportlab.pdfbase.ttfonts import TTFont
+from pathlib import Path
+import sys
+
+def test_chinese_rendering():
+    """Test if Chinese characters can be rendered in PDF"""
+
+    # Font path
+    font_path = "/home/egg/project/Tool_OCR/backend/fonts/NotoSansSC-Regular.ttf"
+
+    # Check if font file exists
+    if not Path(font_path).exists():
+        print(f"❌ Font file not found: {font_path}")
+        return False
+
+    print(f"✓ Font file found: {font_path}")
+
+    try:
+        # Register Chinese font
+        pdfmetrics.registerFont(TTFont('NotoSansSC', font_path))
+        print("✓ Font registered successfully")
+
+        # Create test PDF
+        test_pdf = "/tmp/test_chinese.pdf"
+        c = canvas.Canvas(test_pdf)
+
+        # Set Chinese font
+        c.setFont('NotoSansSC', 14)
+
+        # Draw test text
+        c.drawString(100, 750, "測試中文字符渲染 - Test Chinese Character Rendering")
+        c.drawString(100, 730, "HTD-S1 技術數據表")
+        c.drawString(100, 710, "這是一個 PDF 生成測試")
+
+        c.save()
+        print(f"✓ Test PDF created: {test_pdf}")
+
+        # Check file size
+        file_size = Path(test_pdf).stat().st_size
+        print(f"✓ PDF file size: {file_size} bytes")
+
+        if file_size > 0:
+            print("\n✅ Chinese font rendering test PASSED")
+            return True
+        else:
+            print("\n❌ PDF file is empty")
+            return False
+
+    except Exception as e:
+        print(f"❌ Error during testing: {e}")
+        import traceback
+        traceback.print_exc()
+        return False
+
+if __name__ == "__main__":
+    success = test_chinese_rendering()
+    sys.exit(0 if success else 1)