feat(server): add EnableCoverArtUpload config option

Allow administrators to disable playlist cover art upload/removal for
non-admin users via the new EnableCoverArtUpload config option (default: true).

- Guard uploadPlaylistImage and deletePlaylistImage endpoints (403 for non-admin when disabled)
- Set CoverArtRole in Subsonic GetUser/GetUsers responses based on config and admin status
- Pass config to frontend and conditionally hide upload/remove UI controls
- Admins always retain upload capability regardless of setting

server/subsonic/users.go

@@ -22,6 +22,7 @@ func buildUserResponse(user model.User) responses.User {
 		ScrobblingEnabled: true,
 		DownloadRole:      conf.Server.EnableDownloads,
 		ShareRole:         conf.Server.EnableSharing,
+		CoverArtRole:      conf.Server.EnableCoverArtUpload || user.IsAdmin,
 		Folder:            slice.Map(user.Libraries, func(lib model.Library) int32 { return int32(lib.ID) }),
 	}