Add window persistence, clickable step indicator, file list improvements

pixstrip-core/tests/storage_tests.rs

@@ -160,6 +160,9 @@ fn save_and_load_session() {
             last_output_dir: Some("/home/user/processed".into()),
             last_preset_name: Some("Blog Photos".into()),
             current_step: 3,
+            window_width: Some(1024),
+            window_height: Some(768),
+            window_maximized: false,
         };
 
         session_store.save(&session).unwrap();