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.

Command.h

@@ -1,8 +1,7 @@
 /*
  * Command.h - command model and registry for editor actions
  */
-#ifndef KTE_COMMAND_H
-#define KTE_COMMAND_H
+#pragma once
 
 #include <functional>
 #include <string>
@@ -155,6 +154,4 @@ void InstallDefaultCommands();
 // Returns true if the command executed successfully.
 bool Execute(Editor &ed, CommandId id, const std::string &arg = std::string(), int count = 0);
 
-bool Execute(Editor &ed, const std::string &name, const std::string &arg = std::string(), int count = 0);
-
-#endif // KTE_COMMAND_H
+bool Execute(Editor &ed, const std::string &name, const std::string &arg = std::string(), int count = 0);