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.

src/CMakeLists.txt

@@ -65,4 +65,6 @@ target_sources(${PROJECT_NAME}
         scene/world_scene.cpp
         scene/scene_manager.cpp
         ui/world_ui_manager.cpp
+        ui/main_menu_ui_manager.cpp
+        scene/main_menu_scene.cpp
 )