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/specs/result-export/spec.md

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+# Result Export - Delta Changes
+
+## ADDED Requirements
+
+### Requirement: Export Interface
+The Export page SHALL support downloading OCR results in multiple formats using V2 task APIs.
+
+#### Scenario: Export page uses V2 download endpoints
+- **WHEN** user selects a format and clicks export button
+- **THEN** frontend SHALL call V2 endpoint `/api/v2/tasks/{task_id}/download/{format}`
+- **AND** frontend SHALL NOT call V1 `/api/v2/export` endpoint (which returns 404)
+- **AND** file SHALL download successfully
+
+#### Scenario: Export supports multiple formats
+- **WHEN** user exports a completed task
+- **THEN** system SHALL support downloading as TXT, JSON, Excel, Markdown, and PDF
+- **AND** each format SHALL use correct V2 download endpoint
+- **AND** downloaded files SHALL contain task OCR results
+
+### Requirement: Multi-Task Export Selection
+The Export page SHALL allow users to select and export multiple tasks.
+
+#### Scenario: Select multiple tasks for export
+- **WHEN** Export page loads
+- **THEN** page SHALL display list of user's completed tasks
+- **AND** page SHALL provide checkboxes to select multiple tasks
+- **AND** page SHALL NOT require batch ID from upload store (legacy V1 behavior)
+
+#### Scenario: Export selected tasks
+- **WHEN** user selects multiple tasks and clicks export
+- **THEN** system SHALL download each selected task's results in chosen format
+- **AND** downloaded files SHALL be named distinctly (e.g., `{task_id}_result.{ext}`)
+- **AND** system MAY provide option to download as ZIP archive for multiple files
+
+### Requirement: Export Configuration Persistence
+Export settings (format, thresholds, templates) SHALL apply consistently to V2 task downloads.
+
+#### Scenario: Apply confidence threshold to export
+- **WHEN** user sets confidence threshold to 0.7 and exports
+- **THEN** downloaded results SHALL only include OCR text with confidence >= 0.7
+- **AND** threshold SHALL apply via V2 download endpoint query parameters
+
+#### Scenario: Apply CSS template to PDF export
+- **WHEN** user selects CSS template for PDF format
+- **THEN** downloaded PDF SHALL use selected styling
+- **AND** template SHALL be passed to V2 `/tasks/{id}/download/pdf` endpoint

openspec/changes/archive/2025-11-17-fix-v2-api-ui-issues/specs/task-management/spec.md

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+# Task Management - Delta Changes
+
+## ADDED Requirements
+
+### Requirement: Task Result Generation
+The OCR service SHALL generate both JSON and Markdown result files for completed tasks with actual content.
+
+#### Scenario: Markdown file contains OCR results
+- **WHEN** a task completes OCR processing successfully
+- **THEN** the generated `.md` file SHALL contain the extracted text in markdown format
+- **AND** the file size SHALL be greater than 0 bytes
+- **AND** the markdown SHALL include headings, paragraphs, and formatting based on OCR layout detection
+
+#### Scenario: Result files stored in task directory
+- **WHEN** OCR processing completes for task ID `88c6c2d2-37e1-48fd-a50f-406142987bdf`
+- **THEN** result files SHALL be stored in `storage/results/88c6c2d2-37e1-48fd-a50f-406142987bdf/`
+- **AND** both `<filename>_result.json` and `<filename>_result.md` SHALL exist
+- **AND** both files SHALL contain valid OCR output data
+
+### Requirement: Task Detail View
+The frontend SHALL provide a dedicated page for viewing individual task details.
+
+#### Scenario: Navigate to task detail page
+- **WHEN** user clicks "View Details" button on task in Task History page
+- **THEN** browser SHALL navigate to `/tasks/{task_id}`
+- **AND** TaskDetailPage component SHALL render
+
+#### Scenario: Display task information
+- **WHEN** TaskDetailPage loads for a valid task ID
+- **THEN** page SHALL display task metadata (filename, status, processing time, confidence)
+- **AND** page SHALL show markdown preview of OCR results
+- **AND** page SHALL provide download buttons for JSON, Markdown, and PDF formats
+
+#### Scenario: Download from task detail page
+- **WHEN** user clicks download button for a specific format
+- **THEN** browser SHALL download the file using `/api/v2/tasks/{task_id}/download/{format}` endpoint
+- **AND** downloaded file SHALL contain the task's OCR results in requested format
+
+### Requirement: Results Page V2 Migration
+The Results page SHALL use V2 task-based APIs instead of V1 batch APIs.
+
+#### Scenario: Load task results instead of batch
+- **WHEN** Results page loads with a task ID in upload store
+- **THEN** page SHALL call `apiClientV2.getTask(taskId)` to fetch task details
+- **AND** page SHALL NOT call any V1 batch status endpoints
+- **AND** task information SHALL display correctly
+
+#### Scenario: Handle missing task gracefully
+- **WHEN** Results page loads without a task ID
+- **THEN** page SHALL display helpful message directing user to upload page
+- **AND** page SHALL provide button to navigate to `/upload`