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.

test_undo.cc

@@ -1,10 +1,11 @@
-#include <iostream>
 #include <cassert>
 #include <fstream>
+#include <iostream>
+
+#include "Buffer.h"
+#include "Command.h"
 #include "Editor.h"
 #include "TestFrontend.h"
-#include "Command.h"
-#include "Buffer.h"
 
 
 int