Add preset export/delete buttons and single-instance support

- User preset rows now have export and delete action buttons
- Export opens file save dialog for .pixstrip-preset files
- Delete removes preset from storage and UI
- Single-instance via GIO HANDLES_OPEN for file manager integration
- Screen reader announcements on step navigation

pixstrip-gtk/src/steps/step_workflow.rs

@@ -175,6 +175,53 @@ pub fn build_workflow_page(state: &AppState) -> adw::NavigationPage {
                 .activatable(true)
                 .build();
             row.add_prefix(&gtk::Image::from_icon_name(&preset.icon));
+
+            // Export button
+            let export_btn = gtk::Button::builder()
+                .icon_name("document-save-as-symbolic")
+                .tooltip_text("Export preset")
+                .valign(gtk::Align::Center)
+                .build();
+            export_btn.add_css_class("flat");
+            let preset_for_export = preset.clone();
+            export_btn.connect_clicked(move |btn| {
+                let p = preset_for_export.clone();
+                let dialog = gtk::FileDialog::builder()
+                    .title("Export Preset")
+                    .initial_name(&format!("{}.pixstrip-preset", p.name))
+                    .modal(true)
+                    .build();
+                if let Some(window) = btn.root().and_then(|r| r.downcast::<gtk::Window>().ok()) {
+                    dialog.save(Some(&window), gtk::gio::Cancellable::NONE, move |result| {
+                        if let Ok(file) = result
+                            && let Some(path) = file.path()
+                        {
+                            let store = pixstrip_core::storage::PresetStore::new();
+                            let _ = store.export_to_file(&p, &path);
+                        }
+                    });
+                }
+            });
+            row.add_suffix(&export_btn);
+
+            // Delete button
+            let delete_btn = gtk::Button::builder()
+                .icon_name("user-trash-symbolic")
+                .tooltip_text("Delete preset")
+                .valign(gtk::Align::Center)
+                .build();
+            delete_btn.add_css_class("flat");
+            delete_btn.add_css_class("error");
+            let pname = preset.name.clone();
+            let row_ref = row.clone();
+            let group_ref = user_group.clone();
+            delete_btn.connect_clicked(move |_| {
+                let store = pixstrip_core::storage::PresetStore::new();
+                let _ = store.delete(&pname);
+                group_ref.remove(&row_ref);
+            });
+            row.add_suffix(&delete_btn);
+
             row.add_suffix(&gtk::Image::from_icon_name("go-next-symbolic"));
 
             let jc = state.job_config.clone();