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>

frontend/src/components/ui/checkbox.tsx

@@ -0,0 +1,46 @@
+import * as React from 'react'
+import { Check } from 'lucide-react'
+import { cn } from '@/lib/utils'
+
+export interface CheckboxProps extends Omit<React.InputHTMLAttributes<HTMLInputElement>, 'type'> {
+  checked?: boolean
+  onCheckedChange?: (checked: boolean) => void
+}
+
+const Checkbox = React.forwardRef<HTMLInputElement, CheckboxProps>(
+  ({ className, checked, onCheckedChange, onChange, ...props }, ref) => {
+    const handleChange = (e: React.ChangeEvent<HTMLInputElement>) => {
+      onChange?.(e)
+      onCheckedChange?.(e.target.checked)
+    }
+
+    return (
+      <div className="relative inline-flex items-center">
+        <input
+          type="checkbox"
+          className="sr-only peer"
+          checked={checked}
+          onChange={handleChange}
+          ref={ref}
+          {...props}
+        />
+        <div
+          className={cn(
+            'h-5 w-5 shrink-0 rounded border border-primary shadow transition-smooth',
+            'peer-focus-visible:outline-none peer-focus-visible:ring-2 peer-focus-visible:ring-ring peer-focus-visible:ring-offset-2',
+            'peer-disabled:cursor-not-allowed peer-disabled:opacity-50',
+            'peer-checked:bg-primary peer-checked:text-primary-foreground',
+            'hover:border-primary/80 cursor-pointer',
+            'flex items-center justify-center',
+            className
+          )}
+        >
+          {checked && <Check className="h-4 w-4 text-white" strokeWidth={3} />}
+        </div>
+      </div>
+    )
+  }
+)
+Checkbox.displayName = 'Checkbox'
+
+export { Checkbox }