rfiacne/mixbilibili — reverse-engineered prompt

Build me a simple cross platform command line tool called mixbilibili that helps people clean up Bilibili downloads by merging matching video and audio files in bulk.

It should look in a folder for matching .mp4 and .m4a files with the same name, skip anything that is still downloading with aria2, and use ffmpeg to merge them into finished videos. By default it should work in the current folder, output mkv files, run multiple merges in parallel, and delete the original source files after a successful merge. Let me choose the source folder, output folder, output format, number of jobs, whether to keep source files, dry run, quiet mode, verbose ffmpeg output, resume an interrupted batch, and retry failed merges.

Please make it friendly for normal terminal users on Windows, macOS, and Linux. If ffmpeg isn’t installed, detect that and prompt the user with install guidance. Show useful progress with speed and ETA when possible, but keep output clean in scripts. Also auto switch between English and Chinese based on the system language.