Add onboarding wizard framework and startup hook

src/desktop.py

@@ -34,6 +34,7 @@ class DesktopAdapter(Protocol):
         state_getter: Callable[[], str],
         on_quit: Callable[[], None],
         *,
+        on_setup_wizard: Callable[[], None] | None = None,
         is_paused_getter: Callable[[], bool] | None = None,
         on_toggle_pause: Callable[[], None] | None = None,
         on_reload_config: Callable[[], None] | None = None,