Introduce file picker and GUI configuration with enhancements.

- Add visual file picker for GUI with toggle support.
- Introduce `GUIConfig` class for loading GUI settings from configuration file.
- Refactor window initialization to support dynamic sizing based on configuration.
- Add macOS-specific handling for fullscreen behavior.
- Improve header inclusion order and minor code cleanup.

Frontend.h

@@ -4,8 +4,6 @@
 #ifndef KTE_FRONTEND_H
 #define KTE_FRONTEND_H
 
-#include <memory>
-
 
 class Editor;
 class InputHandler;