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.

TestInputHandler.h

@@ -1,9 +1,7 @@
 /*
  * TestInputHandler.h - programmable input handler for testing
  */
-#ifndef KTE_TEST_INPUT_HANDLER_H
-#define KTE_TEST_INPUT_HANDLER_H
-
+#pragma once
 #include <queue>
 
 #include "InputHandler.h"
@@ -29,6 +27,4 @@ public:
 
 private:
 	std::queue<MappedInput> queue_;
-};
-
-#endif // KTE_TEST_INPUT_HANDLER_H
+};