v10.0.4

Fix GPS coordinates not saved to videos on Macs without ffmpeg

The packaged app bundles PyAV and mutagen but no ffmpeg CLI. On machines
without ffmpeg, video metadata fell back to mutagen, which can only write
iTunes-style freeform ilst atoms - Apple Photos/Finder/Spotlight ignore
those, so downloaded mp4s showed no location despite coordinates being
found and no errors reported.

Add set_video_metadata_pyav: a stream-copy remux via bundled PyAV with
movflags=use_metadata_tags, producing the same QuickTime mdta keys
(com.apple.quicktime.location.ISO6709 etc.) as the ffmpeg path. Metadata
fallback order at all call sites is now ffmpeg -> PyAV -> mutagen, and a
visible warning is logged if GPS data ends up mutagen-only.