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.

UndoSystem.h

@@ -1,6 +1,4 @@
-#ifndef KTE_UNDOSYSTEM_H
-#define KTE_UNDOSYSTEM_H
-
+#pragma once
 #include <string_view>
 #include <cstddef>
 #include <cstdint>
@@ -54,6 +52,4 @@ private:
 	UndoTree &tree_;
 	// Internal hint for Delete batching: whether next Append() should prepend
 	bool pending_prepend_ = false;
-};
-
-#endif // KTE_UNDOSYSTEM_H
+};