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.

Editor.h

@@ -1,9 +1,7 @@
 /*
  * Editor.h - top-level editor state and buffer management
  */
-#ifndef KTE_EDITOR_H
-#define KTE_EDITOR_H
-
+#pragma once
 #include <cstddef>
 #include <ctime>
 #include <string>
@@ -561,6 +559,4 @@ public:
 private:
 	std::string replace_find_tmp_;
 	std::string replace_with_tmp_;
-};
-
-#endif // KTE_EDITOR_H
+};