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.

OptimizedSearch.h

@@ -1,7 +1,5 @@
 // OptimizedSearch.h - Boyer–Moore (bad character) based substring search
-#ifndef KTE_OPTIMIZED_SEARCH_H
-#define KTE_OPTIMIZED_SEARCH_H
-
+#pragma once
 #include <array>
 #include <cstddef>
 #include <string>
@@ -22,6 +20,4 @@ private:
 	std::string last_pat_;
 
 	void build_bad_char(const std::string &pattern);
-};
-
-#endif // KTE_OPTIMIZED_SEARCH_H
+};