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>

.gitea/workflows/ci.yml

@@ -55,5 +55,8 @@ jobs:
         run: powershell -NoProfile -ExecutionPolicy Bypass -File tool/setup_flutter_windows.ps1
       - name: Updater tests
         shell: powershell
-        working-directory: apps/desktop
-        run: flutter test test/updater_test.dart
+        working-directory: packages/core
+        run: |
+          dart pub get
+          if ($LASTEXITCODE) { exit 1 }
+          dart test test/updater_test.dart