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/main.dart

@@ -13,7 +13,7 @@ import 'src/platform/desktop_entry.dart';
 import 'src/screens/home/ui.dart';
 import 'src/state/session_state.dart';
 import 'src/state/settings.dart';
-import 'src/update/updater.dart';
+import 'src/update/update_controller.dart' show launchArgs;
 
 Future<void> main(List<String> args) async {
   launchArgs = List.unmodifiable(args);