fix: surface detailed workflow errors in the UI

[yuefengw]

Description

This PR improves workflow error visibility in the new UI. Instead of only showing a generic failure message, the backend now propagates structured error details through the status API and workflow WebSocket, and the frontend displays the failed stage, error category, task/log references, and actionable troubleshooting hints.

It also fixes a related terminal-state handling issue where pipeline results with status: "error" could be broadcast as completion messages.

Related Issues

Closes #84

Changes Made

• Added error_details to workflow task state, status API responses, and workflow WebSocket error/status messages.
• Classified common workflow failures, including provider/gateway timeouts such as 504 Gateway Time-out.
• Included task identifiers, failed stage, progress, log paths, and log stream URL in structured error details.
• Kept the existing error string field for backward compatibility.
• Added a reusable frontend WorkflowErrorDetails component for Paper-to-Code and Chat Planning error cards.
• Preserved detailed errors during task recovery after page refresh.
• Ensured pipeline results with status: "error" are broadcast as error events, not completion events.
• Added regression tests for structured error details and error-state broadcasting.

Checklist

• Changes tested locally
• Code reviewed
• Documentation updated (if necessary)
• Unit tests added (if applicable)

Additional Notes

Local validation performed:

python -m pytest tests/ui_session_resume_test.py -q
python -m py_compile new_ui/backend/services/workflow_service.py new_ui/backend/api/routes/workflows.py new_ui/backend/api/websockets/workflow_ws.py new_ui/backend/models/responses.py
cd new_ui/frontend && npm run build