Add server endpoint to trigger a manual download for a user-selected candidate from the candidates modal (/api/downloads/task/<id>/download-candidate). The endpoint validates input, resets task and batch state (status, error, used_sources, active_count, permanently_failed_tracks), reconstructs Track/TrackResult objects and dispatches a background download attempt via missing_download_executor. Update the frontend candidates modal to show a download button per candidate, wire it to POST the candidate to the new API, and add CSS for table layout and download button styling. Enables restarting failed/not_found tasks by choosing a specific source without blocking the UI.
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