ui: add main menu scene and interactive button widget

- 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.

include/Cubed/ui/world_ui_manager.hpp

@@ -25,7 +25,7 @@ public:
 private:
     WorldScene& m_scene;
     std::unordered_map<std::string, std::unique_ptr<Widget>> m_widgets;
-
+    bool handle_mouse_move_event(const MouseMoveEvent& e);
     bool handle_mouse_button_event(const MouseButtonEvent& e);
     bool handle_window_resize_event(const WindowResizeEvent& e);
     bool handle_mouse_wheel_event(const MouseWheelEvent& e);