Wire welcome dialog with navigation and first-run detection

- Welcome dialog buttons navigate between pages
- Done button closes dialog and marks first_run_complete
- Show welcome dialog on first launch only
- Add first_run_complete field to AppConfig with serde(default)

pixstrip-gtk/src/app.rs

@@ -159,6 +159,8 @@ fn build_ui(app: &adw::Application) {
     ui.step_indicator.set_current(0);
 
     window.present();
+
+    crate::welcome::show_welcome_if_first_launch(&window);
 }
 
 fn build_menu() -> gtk::gio::Menu {