Replace auto-start after loading with a '开始游戏' button overlay.
The user click provides the required user gesture for autoplay with sound,
fixing the issue where play() was silently rejected and choices never appeared.
- Reorder onEnd callback before play() in Engine.goToScene to prevent
missed ended event if video ends synchronously
- Wait for loadedmetadata event in VideoManager.play() before seeking
to ensure currentTime reset works correctly on new video sources
