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/App.tsx

@@ -6,6 +6,7 @@ import ResultsPage from '@/pages/ResultsPage'
 import ExportPage from '@/pages/ExportPage'
 import SettingsPage from '@/pages/SettingsPage'
 import TaskHistoryPage from '@/pages/TaskHistoryPage'
+import TaskDetailPage from '@/pages/TaskDetailPage'
 import AdminDashboardPage from '@/pages/AdminDashboardPage'
 import AuditLogsPage from '@/pages/AuditLogsPage'
 import Layout from '@/components/Layout'
@@ -32,6 +33,7 @@ function App() {
         <Route path="results" element={<ResultsPage />} />
         <Route path="export" element={<ExportPage />} />
         <Route path="tasks" element={<TaskHistoryPage />} />
+        <Route path="tasks/:taskId" element={<TaskDetailPage />} />
         <Route path="settings" element={<SettingsPage />} />
 
         {/* Admin routes - require admin privileges */}