Updater: update channel Stable / Dev in Settings → Updates; Dev stays on test builds after a release

Dev reads the dev and stable manifests and offers the newer by semver;
a test build stores Dev on its first start, so the release that updates
it keeps the channel. Switching to Stable in a test build asks before
going back to an older release. tool/dev_release.sh's one-time command
also writes the update-channel file that sets Dev.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/app/app_graph.dart

@@ -167,6 +167,7 @@ class AppGraph {
     final updates = UpdateController.instance;
     final whatsNew = WhatsNew(store: SettingsWhatsNewStore(settings));
     updates.beforeRestart ??= whatsNew.installing;
+    updates.channelOf ??= () => settings.updateChannel.peek();
     return AppGraph._(
       ports: p,
       sfx: sfx,