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.

UndoSystem.h

@@ -2,8 +2,10 @@
 #define KTE_UNDOSYSTEM_H
 
 #include <string_view>
+
 #include "UndoTree.h"
 
+
 class Buffer;
 
 class UndoSystem {
@@ -39,7 +41,6 @@ private:
 
 	void update_dirty_flag();
 
-private:
 	Buffer *buf_;
 	UndoTree &tree_;
 	// Internal hint for Delete batching: whether next Append() should prepend