fix: migrate UI to V2 API and fix admin dashboard

Backend fixes:
- Fix markdown generation using correct 'markdown_content' key in tasks.py
- Update admin service to return flat data structure matching frontend types
- Add task_count and failed_tasks fields to user statistics
- Fix top users endpoint to return complete user data

Frontend fixes:
- Migrate ResultsPage from V1 batch API to V2 task API with polling
- Create TaskDetailPage component with markdown preview and download buttons
- Refactor ExportPage to support multi-task selection using V2 download endpoints
- Fix login infinite refresh loop with concurrency control flags
- Create missing Checkbox UI component

New features:
- Add /tasks/:taskId route for task detail view
- Implement multi-task batch export functionality
- Add real-time task status polling (2s interval)

OpenSpec:
- Archive completed proposal 2025-11-17-fix-v2-api-ui-issues
- Create result-export and task-management specifications

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

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

backend/app/routers/tasks.py

@@ -87,7 +87,7 @@ def process_task_ocr(task_id: str, task_db_id: int, file_path: str, filename: st
 
         # Save Markdown result
         markdown_path = result_dir / f"{Path(filename).stem}_result.md"
-        markdown_content = ocr_result.get('markdown', '')
+        markdown_content = ocr_result.get('markdown_content', '')
         with open(markdown_path, 'w', encoding='utf-8') as f:
             f.write(markdown_content)