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>

frontend/src/hooks/useTaskValidation.ts

@@ -34,6 +34,11 @@ export function useTaskValidation(options?: {
 
   const [isNotFound, setIsNotFound] = useState(false)
 
+  // Reset isNotFound when taskId changes (new upload)
+  useEffect(() => {
+    setIsNotFound(false)
+  }, [taskId])
+
   const { data: taskDetail, isLoading, error, isFetching } = useQuery({
     queryKey: ['taskDetail', taskId],
     queryFn: () => apiClientV2.getTask(taskId!),