Respect excluded files in output step counts and processing validation

- Output step now shows only included (non-excluded) image count and size
- Navigate-to-step refresh accounts for exclusions
- Process action checks for included images, not just loaded files

pixstrip-gtk/src/app.rs

@@ -569,18 +569,21 @@ fn navigate_to_step(ui: &WizardUi, target: usize) {
     if target == 9 {
         // Output step - update image count, total size, and operation summary
         let files = ui.state.loaded_files.borrow();
-        let count = files.len();
+        let excluded = ui.state.excluded_files.borrow();
+        let included_count = files.iter().filter(|p| !excluded.contains(*p)).count();
         let total_size: u64 = files.iter()
+            .filter(|p| !excluded.contains(*p))
             .filter_map(|p| std::fs::metadata(p).ok())
             .map(|m| m.len())
             .sum();
+        drop(excluded);
         drop(files);
         if let Some(page) = ui.pages.get(9) {
             walk_widgets(&page.child(), &|widget| {
                 if let Some(row) = widget.downcast_ref::<adw::ActionRow>()
                     && row.title().as_str() == "Images to process"
                 {
-                    row.set_subtitle(&format!("{} images ({})", count, format_bytes(total_size)));
+                    row.set_subtitle(&format!("{} images ({})", included_count, format_bytes(total_size)));
                 }
             });
         }