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/ui/label.cpp

@@ -4,7 +4,7 @@
 #include "Cubed/tools/font.hpp"
 namespace Cubed {
 Label::Label(const std::string& id) : Widget(id) {}
-
+Label::Label() {}
 Label& Label::set_text(std::string_view text) {
     m_text.text = text;
     m_dirty = true;