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.

UndoTree.h

@@ -1,6 +1,4 @@
-#ifndef KTE_UNDOTREE_H
-#define KTE_UNDOTREE_H
-
+#pragma once
 #include "UndoNode.h"
 
 
@@ -9,7 +7,4 @@ struct UndoTree {
 	UndoNode *current = nullptr; // current state of buffer
 	UndoNode *saved   = nullptr; // points to node matching last save (for dirty flag)
 	UndoNode *pending = nullptr; // in-progress batch (detached)
-};
-
-
-#endif // KTE_UNDOTREE_H
+};