One updater for app and server: updater in core; watch-together-server --upgrade / --check-update; update.sh uses the server

The server's version is stamped like the app's (APP_VERSION) when packaged,
so it can compare against the manifest; --upgrade swaps the whole install
folder (app and server) and asks for a restart. update.sh/.bat no longer
need GTK.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/jellyfin/accounts.dart

@@ -8,7 +8,6 @@ import 'package:hive_ce/hive.dart';
 import 'package:sync_relay_core/core.dart';
 
 import '../screens/home/picked_media.dart';
-import '../update/updater.dart' show appVersion;
 
 // Linked Jellyfin servers, one account per ServerId, in their own Hive box.
 //