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-core/src/pipeline.rs

@@ -89,6 +89,19 @@ impl ProcessingJob {
             })
             .unwrap_or("bin");
 
-        self.output_dir.join(format!("{}.{}", stem, ext))
+        let filename = format!("{}.{}", stem, ext);
+
+        if self.preserve_directory_structure {
+            // Maintain relative path from input_dir
+            if let Ok(rel) = source.path.strip_prefix(&self.input_dir) {
+                if let Some(parent) = rel.parent() {
+                    if parent.components().count() > 0 {
+                        return self.output_dir.join(parent).join(filename);
+                    }
+                }
+            }
+        }
+
+        self.output_dir.join(filename)
     }
 }