Wire help button to show contextual step help dialog

Connect the header bar help button to the existing show_step_help
function, showing context-aware help for the current wizard step.
Remove duplicate function definition.

pixstrip-gtk/src/app.rs

@@ -395,6 +395,16 @@ fn build_ui(app: &adw::Application) {
         });
     }
 
+    // Wire help button to show contextual help for current step
+    {
+        let wizard = ui.state.wizard.clone();
+        let window_ref = window.clone();
+        help_button.connect_clicked(move |_| {
+            let step = wizard.borrow().current_step;
+            show_step_help(&window_ref, step);
+        });
+    }
+
     setup_window_actions(&window, &ui);
     update_nav_buttons(
         &ui.state.wizard.borrow(),