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.

GapBuffer.h

@@ -6,6 +6,7 @@
 
 #include <cstddef>
 
+
 class GapBuffer {
 public:
 	GapBuffer();