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/serve_index.go

@@ -61,6 +61,7 @@ func serveIndex(ds model.DataStore, fs fs.FS, shareInfo *model.Share) http.Handl
 			"losslessFormats":           strings.ToUpper(strings.Join(mime.LosslessFormats, ",")),
 			"devActivityPanel":          conf.Server.DevActivityPanel,
 			"enableUserEditing":         conf.Server.EnableUserEditing,
+			"enableCoverArtUpload":      conf.Server.EnableCoverArtUpload,
 			"enableSharing":             conf.Server.EnableSharing,
 			"shareURL":                  conf.Server.ShareURL,
 			"defaultDownloadableShare":  conf.Server.DefaultDownloadableShare,