Allow users to prefer a specific video codec (H.264, H.265, AV1, VP9)
when adding downloads. The selector filters available formats via
yt-dlp format strings, falling back to best available if the preferred
codec is not found. The completed downloads table now shows Quality
and Codec columns.
Display the completion time for each download in the done list.
The backend already stores a nanosecond timestamp on DownloadInfo; this wires it up to the frontend using Angular's DatePipe.
