Add PauseMenuUIManager class and integrate pause state into WorldScene.
Escape key toggles pause in ClientPlayer and PauseMenuUIManager.
Refactor Window mouse and imgui control to support pause state.
- 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.
- Introduce Event variant and Overloaded helper for input handling
- Move game state (camera, client_world, dev_panel) from App to WorldScene
- Add SceneManager with push/pop/change operations for scene stack
- Refactor input callbacks to dispatch events through scene hierarchy
- Extract Argument struct to separate header
- Update Renderer and WorldRenderer to accept world reference
- Replace global input state with event-driven processing in ClientPlayer, Camera, etc.
