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>

frontend/src/pages/TaskDetailPage.tsx

@@ -3,7 +3,7 @@ import { useTranslation } from 'react-i18next'
 import { useQuery } from '@tanstack/react-query'
 import { Button } from '@/components/ui/button'
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card'
-import MarkdownPreview from '@/components/MarkdownPreview'
+import PDFViewer from '@/components/PDFViewer'
 import { useToast } from '@/components/ui/toast'
 import { apiClientV2 } from '@/services/apiV2'
 import {
@@ -149,6 +149,14 @@ export default function TaskDetailPage() {
   const isProcessing = taskDetail.status === 'processing'
   const isFailed = taskDetail.status === 'failed'
 
+  // Construct PDF URL for preview
+  const API_BASE_URL = import.meta.env.VITE_API_BASE_URL || 'http://localhost:8000'
+  const pdfUrl = taskId ? `${API_BASE_URL}/api/v2/tasks/${taskId}/download/pdf` : ''
+
+  // Get auth token for PDF preview
+  const authToken = localStorage.getItem('auth_token_v2')
+  const pdfHttpHeaders = authToken ? { Authorization: `Bearer ${authToken}` } : undefined
+
   return (
     <div className="space-y-6">
       {/* Page Header */}
@@ -329,17 +337,11 @@ export default function TaskDetailPage() {
 
       {/* Result Preview */}
       {isCompleted && (
-        <Card>
-          <CardHeader>
-            <CardTitle>處理結果預覽</CardTitle>
-          </CardHeader>
-          <CardContent>
-            <MarkdownPreview
-              title={`OCR 結果 - ${taskDetail.filename || '未知檔案'}`}
-              content="請使用上方下載按鈕下載 Markdown、JSON 或 PDF 格式查看完整結果"
-            />
-          </CardContent>
-        </Card>
+        <PDFViewer
+          title={`OCR 結果預覽 - ${taskDetail.filename || '未知檔案'}`}
+          pdfUrl={pdfUrl}
+          httpHeaders={pdfHttpHeaders}
+        />
       )}
     </div>
   )