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.

GUIConfig.h

@@ -0,0 +1,27 @@
+/*
+ * GUIConfig - loads simple GUI configuration from $HOME/.config/kte/kge.ini
+ */
+#ifndef KTE_GUI_CONFIG_H
+#define KTE_GUI_CONFIG_H
+
+#include <string>
+
+#ifndef KTE_FONT_SIZE
+#define KTE_FONT_SIZE 16.0f
+#endif
+
+class GUIConfig {
+public:
+	bool fullscreen = false;
+	int columns     = 80;
+	int rows        = 42;
+	float font_size = (float) KTE_FONT_SIZE;
+
+	// Load from default path: $HOME/.config/kte/kge.ini
+	static GUIConfig Load();
+
+	// Load from explicit path. Returns true if file existed and was parsed.
+	bool LoadFromFile(const std::string &path);
+};
+
+#endif // KTE_GUI_CONFIG_H