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>

openspec/changes/archive/2025-11-17-fix-v2-api-ui-issues/proposal.md

@@ -0,0 +1,24 @@
+# Change: Fix V2 API UI Integration Issues
+
+## Why
+After migrating from V1 batch-based architecture to V2 task-based architecture, several UI pages still reference V1 APIs or have incomplete implementations:
+1. Results page (http://127.0.0.1:5173/results) doesn't display task details - uses non-existent V1 `getBatchStatus` API
+2. Task History page markdown downloads produce empty files (0 bytes) - OCR service not generating markdown content
+3. Task History page "View Details" button navigates to `/tasks/{taskId}` route that doesn't exist
+4. Export page (http://127.0.0.1:5173/export) uses non-existent V1 `/api/v2/export` endpoint (404) and lacks multi-task selection
+5. Admin Dashboard page loads but may have permission or API issues
+
+These issues were discovered during testing with task ID: `88c6c2d2-37e1-48fd-a50f-406142987bdf` using file `Henkel-84-1LMISR4 (漢高).pdf`.
+
+## What Changes
+- Migrate ResultsPage from V1 batch API to V2 task API
+- Fix OCR service markdown generation to produce non-empty .md files
+- Add task detail page route and component at `/tasks/:taskId`
+- Update ExportPage to use V2 download endpoints and support multi-task selection
+- Verify and fix Admin Dashboard API integration and permissions
+
+## Impact
+- Affected specs: task-management, result-export
+- Affected code:
+  - Frontend: `src/pages/ResultsPage.tsx`, `src/pages/ExportPage.tsx`, `src/App.tsx` (routes), new `src/pages/TaskDetailPage.tsx`
+  - Backend: `app/services/ocr_service.py` (markdown generation), `app/routers/tasks.py` (download endpoints)