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.

GUIRenderer.h

@@ -1,9 +1,7 @@
 /*
  * GUIRenderer - ImGui-based renderer for GUI mode
  */
-#ifndef KTE_GUI_RENDERER_H
-#define KTE_GUI_RENDERER_H
-
+#pragma once
 #include "Renderer.h"
 
 class GUIRenderer final : public Renderer {
@@ -13,6 +11,4 @@ public:
 	~GUIRenderer() override = default;
 
 	void Draw(Editor &ed) override;
-};
-
-#endif // KTE_GUI_RENDERER_H
+};