Add window persistence, clickable step indicator, file list improvements

- Window size/position remembered between sessions via SessionStore
- Step indicator dots now clickable to navigate directly to that step,
  with keyboard shortcut hints in tooltips
- File list in add-files dialog shows format and size per image,
  header shows total count and total size
- Welcome dialog now saves skill level choice to config
- SessionState extended with window_width, window_height, window_maximized

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();