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/pubspec.yaml

@@ -23,7 +23,6 @@ dependencies:
   hive_ce_flutter: ^2.3.4
   image: ^4.9.2
   window_manager: ^0.5.2
-  archive: ^4.0.7
   crypto: ^3.0.6
   # Drag & drop and clipboard files/images (vendored desktop_drop + pasteboard).
   fox_utils: