KeysPage::new spawns the connect-and-render flow on glib::MainContext::
spawn_local. On success it shows a placeholder PreferencesPage (Task 5
fills in the list); on failure it shows StatusPage with a Retry button
that re-runs the connect attempt.
main.rs installs the multi-thread tokio runtime guard before app.run()
so zbus's tokio futures execute correctly when polled by glib.
Layout deviation from plan: ToolbarView is libadwaita v1_4-gated; the
project sticks with gtk::Box vertical for portability (commit c6a5e94
established this pattern).
5d6b7ceeb0