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/services/admin_service.py

@@ -86,22 +86,16 @@ class AdminService:
         ).count()
 
         return {
-            "users": {
-                "total": total_users,
-                "active": active_users,
-                "active_30d": active_users_30d
-            },
-            "tasks": {
-                "total": total_tasks,
-                "by_status": tasks_by_status,
-                "recent_7d": recent_tasks
-            },
-            "sessions": {
-                "active": active_sessions
-            },
-            "activity": {
-                "logins_7d": recent_logins,
-                "tasks_7d": recent_tasks
+            "total_users": total_users,
+            "active_users": active_users,
+            "total_tasks": total_tasks,
+            "total_sessions": active_sessions,
+            "recent_activity_count": recent_tasks,
+            "task_stats": {
+                "pending": tasks_by_status.get("pending", 0),
+                "processing": tasks_by_status.get("processing", 0),
+                "completed": tasks_by_status.get("completed", 0),
+                "failed": tasks_by_status.get("failed", 0)
             }
         }
 
@@ -142,6 +136,14 @@ class AdminService:
                 )
             ).count()
 
+            # Count failed tasks
+            failed_tasks = db.query(Task).filter(
+                and_(
+                    Task.user_id == user.id,
+                    Task.status == TaskStatus.FAILED
+                )
+            ).count()
+
             # Count active sessions
             active_sessions = db.query(UserSession).filter(
                 and_(
@@ -152,8 +154,9 @@ class AdminService:
 
             user_list.append({
                 **user.to_dict(),
-                "total_tasks": task_count,
+                "task_count": task_count,
                 "completed_tasks": completed_tasks,
+                "failed_tasks": failed_tasks,
                 "active_sessions": active_sessions,
                 "is_admin": self.is_admin(user.email)
             })
@@ -177,34 +180,34 @@ class AdminService:
         Returns:
             List of top users with counts
         """
-        if metric == "completed_tasks":
-            # Top users by completed tasks
-            results = db.query(
-                User,
-                func.count(Task.id).label("task_count")
-            ).join(Task).filter(
-                Task.status == TaskStatus.COMPLETED
-            ).group_by(User.id).order_by(
-                func.count(Task.id).desc()
-            ).limit(limit).all()
-        else:
-            # Top users by total tasks (default)
-            results = db.query(
-                User,
-                func.count(Task.id).label("task_count")
-            ).join(Task).group_by(User.id).order_by(
-                func.count(Task.id).desc()
-            ).limit(limit).all()
+        # Get top users by total tasks
+        results = db.query(
+            User,
+            func.count(Task.id).label("task_count")
+        ).join(Task).group_by(User.id).order_by(
+            func.count(Task.id).desc()
+        ).limit(limit).all()
 
-        return [
-            {
+        # Build result list with both task_count and completed_tasks
+        top_users = []
+        for user, task_count in results:
+            # Count completed tasks for this user
+            completed_tasks = db.query(Task).filter(
+                and_(
+                    Task.user_id == user.id,
+                    Task.status == TaskStatus.COMPLETED
+                )
+            ).count()
+
+            top_users.append({
                 "user_id": user.id,
                 "email": user.email,
                 "display_name": user.display_name,
-                "count": count
-            }
-            for user, count in results
-        ]
+                "task_count": task_count,
+                "completed_tasks": completed_tasks
+            })
+
+        return top_users
 
 
 # Singleton instance