[Feature Request] Custom Playlist Cover Images

[ToxicWaste11]

I'd love the ability to upload custom cover images for playlists in Liner.

Current behavior:

• Playlists automatically display a collage of their tracks' album artwork.
• There's no option to change the playlist cover manually.
• Imported Spotify playlists don't retain their original custom artwork.

Requested features:

1. Allow users to upload custom JPG, PNG, or WebP playlist covers.
2. Add a "Change Cover" option when clicking or right-clicking playlist artwork.
3. Preserve original custom covers when importing playlists from Spotify.
4. Allow users to reset their covers to the automatically generated collage.
5. Keep uploaded covers synchronized across devices where supported.

Why this matters:

Many users carefully customize their playlists with artwork matching their music and aesthetic. Losing those covers when migrating from Spotify makes the library feel incomplete.

This would be a fantastic quality-of-life improvement for Liner!