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_workflow.rs

@@ -36,6 +36,11 @@ pub fn build_workflow_page() -> adw::NavigationPage {
         builtin_flow.append(&card);
     }
 
+    // When a preset card is activated, advance to the next step
+    builtin_flow.connect_child_activated(|flow, _child| {
+        flow.activate_action("win.next-step", None).ok();
+    });
+
     builtin_group.add(&builtin_flow);
     content.append(&builtin_group);
 
@@ -52,6 +57,9 @@ pub fn build_workflow_page() -> adw::NavigationPage {
         .min_children_per_line(2)
         .build();
     custom_flow.append(&custom_card);
+    custom_flow.connect_child_activated(|flow, _child| {
+        flow.activate_action("win.next-step", None).ok();
+    });
     custom_group.add(&custom_flow);
     content.append(&custom_group);