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.

GapBuffer.h

@@ -1,9 +1,7 @@
 /*
  * GapBuffer.h - C++ replacement for abuf append/prepend buffer utilities
  */
-#ifndef KTE_GAPBUFFER_H
-#define KTE_GAPBUFFER_H
-
+#pragma once
 #include <cstddef>
 
 
@@ -75,6 +73,4 @@ private:
 	char *buffer_         = nullptr;
 	std::size_t size_     = 0; // number of valid bytes (excluding terminator)
 	std::size_t capacity_ = 0; // capacity of buffer_ excluding space for terminator
-};
-
-#endif // KTE_GAPBUFFER_H
+};