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/routers/translate.py

@@ -637,17 +637,31 @@ async def download_translated_pdf(
     # Validate format parameter
     use_layout = format.lower() == 'layout'
 
-    # Generate translated PDF to temp file
+    # Generate translated PDF to task result folder (not temp)
+    # Use base name from result JSON (e.g., "scan" or "edit")
+    result_dir = result_json_path.parent
+    base_name = result_json_path.stem.replace('_result', '')
     format_suffix = '_layout' if use_layout else '_reflow'
-    output_filename = f"{task_id}_translated_{lang}{format_suffix}.pdf"
+    output_filename = f"{base_name}_translated_{lang}{format_suffix}.pdf"
+    output_path = result_dir / output_filename
 
-    with tempfile.NamedTemporaryFile(suffix='.pdf', delete=False) as tmp_file:
-        output_path = Path(tmp_file.name)
+    # Check if PDF already exists and is newer than translation JSON
+    if output_path.exists():
+        pdf_mtime = output_path.stat().st_mtime
+        translation_mtime = translation_file.stat().st_mtime
+        if pdf_mtime >= translation_mtime:
+            # PDF is up-to-date, serve directly
+            logger.info(f"Serving cached translated PDF: {output_path}")
+            return FileResponse(
+                path=str(output_path),
+                filename=output_filename,
+                media_type="application/pdf",
+                headers={
+                    "Content-Disposition": f'attachment; filename="{output_filename}"'
+                }
+            )
 
     try:
-        # Use result_dir as image source (contains extracted images)
-        image_dir = result_json_path.parent
-
         # Choose PDF generation method based on format
         if use_layout:
             # Layout mode: preserve original positions with text wrapping
@@ -655,7 +669,7 @@ async def download_translated_pdf(
                 result_json_path=result_json_path,
                 translation_json_path=translation_file,
                 output_path=output_path,
-                source_file_path=image_dir
+                source_file_path=result_dir
             )
         else:
             # Reflow mode: flowing layout
@@ -663,7 +677,7 @@ async def download_translated_pdf(
                 result_json_path=result_json_path,
                 translation_json_path=translation_file,
                 output_path=output_path,
-                source_file_path=image_dir
+                source_file_path=result_dir
             )
 
         if not success:
@@ -672,7 +686,7 @@ async def download_translated_pdf(
                 detail="Failed to generate translated PDF"
             )
 
-        logger.info(f"Generated translated PDF for task {task_id}, lang={lang}")
+        logger.info(f"Generated translated PDF: {output_path}")
 
         return FileResponse(
             path=str(output_path),
@@ -684,14 +698,8 @@ async def download_translated_pdf(
         )
 
     except HTTPException:
-        # Clean up temp file on HTTP errors
-        if output_path.exists():
-            output_path.unlink()
         raise
     except Exception as e:
-        # Clean up temp file on unexpected errors
-        if output_path.exists():
-            output_path.unlink()
         logger.exception(f"Failed to generate translated PDF for task {task_id}")
         raise HTTPException(
             status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,