Improve UX, add popover tour, metadata, and hicolor icons

- Redesign tutorial tour from modal dialogs to popovers pointing at actual UI elements
- Add beginner-friendly improvements: help buttons, tooltips, welcome wizard enhancements
- Add AppStream metainfo with screenshots, branding, categories, keywords, provides
- Update desktop file with GTK category and SingleMainWindow
- Add hicolor icon theme with all sizes (16-512px)
- Fix debounce SourceId panic in rename step
- Various step UI improvements and bug fixes

pixstrip-gtk/src/steps/step_images.rs

@@ -500,6 +500,7 @@ fn build_empty_state() -> gtk::Box {
         .pixel_size(64)
         .css_classes(["dim-label"])
         .build();
+    icon.set_accessible_role(gtk::AccessibleRole::Presentation);
 
     let title = gtk::Label::builder()
         .label("Drop images here")
@@ -529,6 +530,16 @@ fn build_empty_state() -> gtk::Box {
     browse_button.add_css_class("suggested-action");
     browse_button.add_css_class("pill");
 
+    let hint = gtk::Label::builder()
+        .label("Start by adding your images below, then use the Next button to configure each processing step.")
+        .css_classes(["dim-label", "caption"])
+        .halign(gtk::Align::Center)
+        .justify(gtk::Justification::Center)
+        .wrap(true)
+        .margin_bottom(8)
+        .build();
+
+    inner.append(&hint);
     inner.append(&icon);
     inner.append(&title);
     inner.append(&subtitle);
@@ -778,11 +789,19 @@ fn build_loaded_state(state: &AppState) -> gtk::Box {
                 }
             });
 
+            // Set accessible label on thumbnail picture
+            picture.update_property(&[
+                gtk::accessible::Property::Label(&format!("Thumbnail of {}", file_name)),
+            ]);
+
             // Set checkbox state
             let check = find_check_button(overlay.upcast_ref::<gtk::Widget>());
             if let Some(ref check) = check {
                 let is_excluded = excluded.borrow().contains(&path);
                 check.set_active(!is_excluded);
+                check.update_property(&[
+                    gtk::accessible::Property::Label(&format!("Include {} in processing", file_name)),
+                ]);
 
                 // Wire checkbox toggle
                 let excl = excluded.clone();