Add accessible value updates to processing progress bar

Screen readers now announce progress bar value with descriptive
text like "Processing 5 of 47: sunset.jpg" during batch processing,
instead of just reporting the fraction.

pixstrip-gtk/src/app.rs

@@ -1258,8 +1258,15 @@ fn run_processing(_window: &adw::ApplicationWindow, ui: &WizardUi) {
                     file,
                 } => {
                     if let Some(ref bar) = progress_bar {
-                        bar.set_fraction(current as f64 / total as f64);
+                        let frac = current as f64 / total as f64;
+                        bar.set_fraction(frac);
                         bar.set_text(Some(&format!("{}/{} - {}", current, total, file)));
+                        bar.update_property(&[
+                            gtk::accessible::Property::ValueNow(frac * 100.0),
+                            gtk::accessible::Property::ValueText(
+                                &format!("Processing {} of {}: {}", current, total, file)
+                            ),
+                        ]);
                     }
                     let eta = calculate_eta(&start_time, current, total);
                     update_progress_labels(&ui_for_rx.nav_view, current, total, &file, &eta);