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_images.rs

@@ -72,6 +72,7 @@ fn build_empty_state() -> gtk::Box {
         .label("Browse Files")
         .tooltip_text("Add image files (Ctrl+O)")
         .halign(gtk::Align::Center)
+        .action_name("win.add-files")
         .build();
     browse_button.add_css_class("suggested-action");
     browse_button.add_css_class("pill");
@@ -112,6 +113,7 @@ fn build_loaded_state() -> gtk::Box {
     let add_button = gtk::Button::builder()
         .icon_name("list-add-symbolic")
         .tooltip_text("Add more images")
+        .action_name("win.add-files")
         .build();
     add_button.add_css_class("flat");