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.

UndoNode.h

@@ -1,6 +1,4 @@
-#ifndef KTE_UNDONODE_H
-#define KTE_UNDONODE_H
-
+#pragma once
 #include <cstdint>
 #include <string>
 
@@ -20,7 +18,4 @@ struct UndoNode {
 	std::string text;
 	UndoNode *child = nullptr; // next in current timeline
 	UndoNode *next  = nullptr; // redo branch
-};
-
-
-#endif // KTE_UNDONODE_H
+};