feat: optimize task file generation and add visualization download

Backend changes:
- Disable PP-Structure debug file generation by default
- Separate raw_ocr_regions.json generation from debug flag (critical file)
- Add visualization folder download endpoint as ZIP
- Add has_visualization field to TaskDetailResponse
- Stop generating Markdown files
- Save translated PDFs to task folder with caching

Frontend changes:
- Replace JSON/MD download buttons with PDF buttons in TaskHistoryPage
- Add visualization download button in TaskDetailPage
- Fix Processing page task switching issue (reset isNotFound)

Archives two OpenSpec proposals:
- optimize-task-files-and-visualization
- simplify-frontend-add-billing

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/ocr_service.py

@@ -1510,14 +1510,25 @@ class OCRService:
                     'height': ocr_height
                 }]
 
-                # Generate PP-StructureV3 debug outputs if enabled
+                # Always save raw_ocr_regions.json (required for PDF generation and translation)
+                if output_dir:
+                    try:
+                        import json
+                        ocr_json_path = output_dir / f"{image_path.stem}_raw_ocr_regions.json"
+                        with open(ocr_json_path, 'w', encoding='utf-8') as f:
+                            json.dump(text_regions, f, ensure_ascii=False, indent=2)
+                        logger.info(f"Saved raw OCR regions to {ocr_json_path}")
+                    except Exception as ocr_save_error:
+                        logger.warning(f"Failed to save raw OCR regions: {ocr_save_error}")
+
+                # Generate PP-StructureV3 debug outputs if enabled (debug files only)
                 if settings.pp_structure_debug_enabled and output_dir:
                     try:
                         from app.services.pp_structure_debug import PPStructureDebug
                         debug_service = PPStructureDebug(output_dir)
 
-                        # Save raw results as JSON
-                        debug_service.save_raw_results(
+                        # Save PP-Structure raw results and summary (debug only)
+                        debug_service.save_debug_results(
                             pp_structure_results={
                                 'elements': layout_data.get('elements', []),
                                 'total_elements': layout_data.get('total_elements', 0),
@@ -2536,7 +2547,7 @@ class OCRService:
         source_file_path: Optional[Path] = None
     ) -> Tuple[Optional[Path], Optional[Path], Optional[Path]]:
         """
-        Save OCR results to JSON, Markdown, and layout-preserving PDF files
+        Save OCR results to JSON and layout-preserving PDF files
 
         Args:
             result: OCR result (UnifiedDocument or dictionary)
@@ -2546,9 +2557,11 @@ class OCRService:
 
         Returns:
             Tuple of (json_path, markdown_path, pdf_path)
+            Note: markdown_path is always None (Markdown generation removed)
         """
         try:
             output_dir.mkdir(parents=True, exist_ok=True)
+            markdown_path = None  # Markdown generation removed
 
             # Use UnifiedDocumentExporter for standardized export
             if isinstance(result, UnifiedDocument) and UnifiedDocumentExporter is not None:
@@ -2560,31 +2573,16 @@ class OCRService:
                     include_metadata=True,
                     include_statistics=True
                 )
-
-                markdown_path = output_dir / f"{file_id}_output.md"
-                UnifiedDocumentExporter.export_to_markdown(
-                    result,
-                    markdown_path,
-                    include_metadata_header=False  # Keep output clean
-                )
-
-                markdown_content = result.extract_all_text()
             else:
                 # Legacy path for dict results
                 result_dict = result if isinstance(result, dict) else result.to_dict()
-                markdown_content = result.get('markdown_content', '') if isinstance(result, dict) else ''
 
                 # Save JSON
                 json_path = output_dir / f"{file_id}_result.json"
                 with open(json_path, 'w', encoding='utf-8') as f:
                     json.dump(result_dict, f, ensure_ascii=False, indent=2)
 
-                # Save Markdown
-                markdown_path = output_dir / f"{file_id}_output.md"
-                with open(markdown_path, 'w', encoding='utf-8') as f:
-                    f.write(markdown_content)
-
-            logger.info(f"Results saved: {json_path.name}, {markdown_path.name}")
+            logger.info(f"Results saved: {json_path.name}")
 
             # Generate layout-preserving PDF
             pdf_path = None