Add completion sound, preserve directory structure support

- Play system notification sound via canberra-gtk-play when enabled
- Implement preserve_directory_structure in output_path_for to maintain
  relative paths from input directory in output

pixstrip-gtk/src/app.rs

@@ -1682,6 +1682,17 @@ fn show_results(
         }
     }
 
+    // Play completion sound (if enabled in settings)
+    if config.play_completion_sound {
+        std::thread::spawn(|| {
+            // Use canberra-gtk-play for system sound (standard on GNOME)
+            let _ = std::process::Command::new("canberra-gtk-play")
+                .arg("--id=complete")
+                .arg("--description=Processing complete")
+                .output();
+        });
+    }
+
     // Auto-open output folder if enabled
     if config.auto_open_output {
         let output = ui.state.output_dir.borrow().clone();