Wire up step buttons: Browse, preset cards, output directory picker

- Browse Files button triggers win.add-files action
- Add More button in loaded state triggers win.add-files action
- Preset card activation (click) advances to next wizard step
- Custom workflow card activation advances to next step
- Choose output folder button opens folder dialog
- Output step shows current image count when navigated to
- Clean up dead code in update_count_in_box

pixstrip-gtk/src/steps/step_output.rs

@@ -46,6 +46,7 @@ pub fn build_output_page() -> adw::NavigationPage {
         .icon_name("folder-open-symbolic")
         .tooltip_text("Choose output folder")
         .valign(gtk::Align::Center)
+        .action_name("win.choose-output")
         .build();
     choose_button.add_css_class("flat");
     output_row.add_suffix(&choose_button);