Dashboard: cannot delete cron jobs

## Problem
The dashboard displays scheduled cron jobs but there is no way to delete/remove a cron job from the UI. Users have to fall back to the CLI/API to remove a job.

## Expected behavior
- Each cron job row/card has a delete action (with a confirmation prompt to avoid accidental deletion).
- Deleting a job in the dashboard actually removes it from the scheduler (calls the backend's remove endpoint) and updates the UI list immediately.
- Errors from the backend (e.g. job not found) are surfaced to the user instead of failing silently.

Look for the dashboard component that lists cron jobs, likely in a file like `src/components/CronJobList.tsx` or similar. Identify the backend endpoint for removing a cron job (probably a DELETE request). Add a delete button to each job row, implement the API call, handle success/error responses, and update the UI state. Test by running the dashboard and verifying jobs can be deleted with confirmation.

索引モデルが issue の本文から書いたものです。