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.cc

@@ -1,9 +1,10 @@
-#include "GapBuffer.h"
-
 #include <algorithm>
 #include <cassert>
 #include <cstring>
 
+#include "GapBuffer.h"
+
+
 GapBuffer::GapBuffer() = default;