Introduce file picker and GUI configuration with enhancements.

- Add visual file picker for GUI with toggle support.
- Introduce `GUIConfig` class for loading GUI settings from configuration file.
- Refactor window initialization to support dynamic sizing based on configuration.
- Add macOS-specific handling for fullscreen behavior.
- Improve header inclusion order and minor code cleanup.

TerminalFrontend.h

@@ -4,10 +4,11 @@
 #ifndef KTE_TERMINAL_FRONTEND_H
 #define KTE_TERMINAL_FRONTEND_H
 
+#include <termios.h>
+
 #include "Frontend.h"
 #include "TerminalInputHandler.h"
 #include "TerminalRenderer.h"
-#include <termios.h>
 
 
 class TerminalFrontend final : public Frontend {