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.

GUIConfig.h

@@ -1,8 +1,7 @@
 /*
  * GUIConfig - loads simple GUI configuration from $HOME/.config/kte/kge.ini
  */
-#ifndef KTE_GUI_CONFIG_H
-#define KTE_GUI_CONFIG_H
+#pragma once
 
 #include <string>
 
@@ -31,6 +30,4 @@ public:
 
 	// Load from explicit path. Returns true if file existed and was parsed.
 	bool LoadFromFile(const std::string &path);
-};
-
-#endif // KTE_GUI_CONFIG_H
+};