Add accessible labels to processing progress bar, results icon, drop zone

- Progress bar gets "Processing progress" accessible label
- Results success icon gets "Success" accessible label
- Image drop zone gets descriptive accessible label for screen readers

pixstrip-gtk/src/steps/step_images.rs

@@ -306,6 +306,9 @@ fn build_empty_state() -> gtk::Box {
         .margin_end(48)
         .build();
     drop_zone.add_css_class("card");
+    drop_zone.update_property(&[
+        gtk::accessible::Property::Label("Image drop zone. Drop images here or use the Browse Files button."),
+    ]);
 
     let inner = gtk::Box::builder()
         .orientation(gtk::Orientation::Vertical)