Replace set_position with anchor and offset for relative positioning. Add parent pointer to widget hierarchy. Remove manual resize logic in UI managers.
Remove scale from base Widget class and add per-type set_scale/scale methods to Button, Image, and Label. Dispatch separate WindowResizeEvent alongside existing FrameBufferResizeEvent. Correct centering calculations from `+` to `-` in main menu and world UI managers.
- Implement MainMenuScene with a "Start Game" button that pushes the WorldScene.
- Extend Button class with background/foreground, hover detection, and size queries.
- Add virtual width/height and event handlers to Widget base class.
- Update WorldUIManager to propagate mouse move events.
- Adjust Window mouse mode based on game running state.
