video_codec 'none' (REMOVE_VIDEO) drops the video stream (-vn, audio
required); an audio-only source is audio-only output whatever the video
target says, and a video target on it is refused. Audio output gets an
audio container (m4a/ogg/mp3) checked against the codec it will hold, and
a video container asked for audio output is replaced by the codec's
natural one. faststart only for mp4/m4a. Verification learned the
audio-only shape and ffprobe's container names (mkv reports matroska).
The modal offers 'Remove video (audio only)', switches to audio
containers and follows the audio codec, reads Extract Audio, and opens
for audio files (no video codec select).
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
