Code quality, safety, stability, and cleanups.

- Replace header include guards with `#pragma once` and perform minor optimizations.
- Replaced traditional include guards with `#pragma once` for simplicity and to reduce boilerplate in all headers.
- Improved CLI line number handling with clamping and error messaging.
- Enhanced `chdir` error handling for macOS GUI builds.
- Removed redundant logic for GUI builds.
- Adjusted font constructor and registry to handle `const` data pointers consistently.

AppendBuffer.h

@@ -1,8 +1,7 @@
 /*
  * AppendBuffer.h - selector header to choose GapBuffer or PieceTable
  */
-#ifndef KTE_APPENDBUFFER_H
-#define KTE_APPENDBUFFER_H
+#pragma once
 
 #ifdef KTE_USE_PIECE_TABLE
 #include "PieceTable.h"
@@ -10,6 +9,4 @@ using AppendBuffer = PieceTable;
 #else
 #include "GapBuffer.h"
 using AppendBuffer = GapBuffer;
-#endif
-
-#endif // KTE_APPENDBUFFER_H
+#endif