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.

GUIInputHandler.h

@@ -1,9 +1,7 @@
 /*
  * GUIInputHandler - ImGui/SDL2-based input mapping for GUI mode
  */
-#ifndef KTE_GUI_INPUT_HANDLER_H
-#define KTE_GUI_INPUT_HANDLER_H
-
+#pragma once
 #include <mutex>
 #include <queue>
 
@@ -41,6 +39,4 @@ private:
 	bool uarg_had_digits_ = false; // whether any digits were supplied
 	int uarg_value_       = 0; // current absolute value (>=0)
 	std::string uarg_text_; // raw digits/minus typed for status display
-};
-
-#endif // KTE_GUI_INPUT_HANDLER_H
+};