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/admin.py

@@ -91,10 +91,7 @@ async def get_top_users(
     """
     try:
         top_users = admin_service.get_top_users(db, metric=metric, limit=limit)
-        return {
-            "metric": metric,
-            "users": top_users
-        }
+        return top_users
 
     except Exception as e:
         logger.exception("Failed to get top users")