feat(tasks): download task output log from detail page

[Hemil-Sangani]

Task

Todo 8.3 - Download output

Summary

This PR adds the ability to download a task's raw output log directly from the task detail page.

A new backend endpoint serves the task's output.log file, while the frontend adds a Download action to the task header for quick access to the complete log output.

Changes

• Added a new endpoint: GET /api/tasks/<task_id>/output/download
• Validates task existence using TaskReader
• Returns a 404 response when the task or output file is not found
• Serves output.log as a downloadable text/plain attachment
• Names downloaded files as <task_id>_output.log
• Added a Download button to the task detail page header
• Uses the browser's native download behavior without additional frontend requests
• Places the Download action alongside the existing Kill and Re-run actions

Notes

The task detail page displays only the most recent 200 log lines, while the downloaded file contains the complete task output.