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.

PieceTable.h

@@ -1,9 +1,7 @@
 /*
  * PieceTable.h - Alternative to GapBuffer using a piece table representation
  */
-#ifndef KTE_PIECETABLE_H
-#define KTE_PIECETABLE_H
-
+#pragma once
 #include <cstddef>
 #include <string>
 #include <vector>
@@ -93,6 +91,4 @@ private:
 	mutable std::string materialized_;
 	mutable bool dirty_     = true;
 	std::size_t total_size_ = 0;
-};
-
-#endif // KTE_PIECETABLE_H
+};