Replace custom shortcuts dialog with GtkShortcutsWindow, fix Process More button

- Use proper GtkShortcutsWindow with ShortcutsSection/Group/Shortcut widgets
  instead of custom AdwDialog with ActionRows
- Hide step indicator during processing and results screens
- Fix "Process More" button re-triggering processing instead of resetting wizard
- Add accessible label to resize step size preview DrawingArea

pixstrip-gtk/src/steps/step_resize.rs

@@ -242,6 +242,9 @@ pub fn build_resize_page(state: &AppState) -> adw::NavigationPage {
         .margin_top(8)
         .margin_bottom(8)
         .build();
+    drawing.update_property(&[
+        gtk::accessible::Property::Label("Visual comparison of original and target image dimensions"),
+    ]);
 
     let pw = preview_width.clone();
     let ph = preview_height.clone();