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.lock

@@ -10,7 +10,7 @@ packages:
     source: hosted
     version: "1.1.1"
   archive:
-    dependency: "direct main"
+    dependency: transitive
     description:
       name: archive
       sha256: ace891da0862b0e4cabbb064ee3fd87b2728b898949fdb366d83fe98342c9f19