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>

packages/core/pubspec.yaml

@@ -7,6 +7,7 @@ dependencies:
   logging: ^1.2.0
   yaml: ^3.1.0
   path: ^1.9.0
+  archive: ^4.0.7
   crypto: ^3.0.0
   web_socket_channel: ^3.0.0
 dev_dependencies: