Add tutorial overlay tour after welcome wizard

Semi-transparent dialog-based tour with 6 stops covering step
indicator, workflow selection, image adding, navigation, menu,
and a final ready message. Skippable at any time. State persisted
via tutorial_complete flag in AppConfig.

pixstrip-gtk/src/settings.rs

@@ -291,6 +291,7 @@ pub fn build_settings_dialog() -> adw::PreferencesDialog {
     dialog.connect_closed(move |_| {
         let new_config = AppConfig {
             first_run_complete: true,
+            tutorial_complete: true, // preserve if settings are being saved
             output_subfolder: subfolder_row.text().to_string(),
             output_fixed_path: None,
             overwrite_behavior: match overwrite_row.selected() {