Incorporate feedback from Flathub reviewer (yakushabb):
- Use <developer id> block instead of deprecated <developer_name>
- Remove <categories> and <keywords> (pulled from .desktop file)
Expand feature list: local library, Plex integration, immersive and
miniplayer modes, artist blacklist, ListenBrainz, smart cache, and
customization options. Sync both metainfo copies.
- Desktop entry file
- AppStream metainfo with screenshots
- Screenshot images for Flathub listing
These files are required by Flathub reviewers to be in upstream.