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.

UndoNodePool.h

@@ -1,6 +1,4 @@
-#ifndef KTE_UNDONODEPOOL_H
-#define KTE_UNDONODEPOOL_H
-
+#pragma once
 #include <stack>
 #include <vector>
 #include <memory>
@@ -60,6 +58,4 @@ private:
 	std::size_t block_size_;
 	std::vector<std::unique_ptr<UndoNode[]> > blocks_;
 	std::stack<UndoNode *> available_;
-};
-
-#endif // KTE_UNDONODEPOOL_H
+};