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

conf/configuration.go

@@ -76,6 +76,7 @@ type configOptions struct {
 	EnableFavourites                bool
 	EnableStarRating                bool
 	EnableUserEditing               bool
+	EnableCoverArtUpload            bool
 	EnableSharing                   bool
 	ShareURL                        string
 	DefaultShareExpiration          time.Duration
@@ -668,6 +669,7 @@ func setViperDefaults() {
 	viper.SetDefault("enablereplaygain", true)
 	viper.SetDefault("enablecoveranimation", true)
 	viper.SetDefault("enablenowplaying", true)
+	viper.SetDefault("enablecoverartupload", true)
 	viper.SetDefault("enablesharing", false)
 	viper.SetDefault("shareurl", "")
 	viper.SetDefault("defaultshareexpiration", 8760*time.Hour)