[feature] Custom playlist cover art

[Dazezh]

Currently, Music automatically generates cover art for playlists based on the contained albums. This is convenient, but it would be great to have more control for personalization.

It would be a great feature if users could set their own custom image for a playlist. This would allow for:

• Setting a unique image that better represents the playlist's mood or theme.
• Replacing the auto-generated collage with a cleaner, custom design.
• Easier visual identification of playlists in the overview.