feat(main_menu): add exit button and refactor layout with ColumnLayout

- Add `should_close_window()` method to force window close from UI.
- Replace standalone button with ColumnLayout for proper centering.
- Add "Exit" button that calls `should_close_window()`.
- Update ESC key handling to only close window when game is not running.
- Fix ColumnLayout to respect parent's anchor instead of hardcoded TOP_LEFT.
- Remove unused parameter name in Image::update for consistency.

include/Cubed/window.hpp

@@ -32,6 +32,8 @@ public:
 
     void set_game_running(bool running);
 
+    void should_close_window();
+
 private:
     bool m_mouse_enable = false;
     bool m_imgui_init = false;