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.cc

@@ -0,0 +1,107 @@
+#include <cstdlib>
+#include <cstring>
+#include <fstream>
+#include <sstream>
+#include <algorithm>
+
+#include "GUIConfig.h"
+
+
+static void
+trim(std::string &s)
+{
+	auto not_space = [](int ch) {
+		return !std::isspace(ch);
+	};
+	s.erase(s.begin(), std::find_if(s.begin(), s.end(), not_space));
+	s.erase(std::find_if(s.rbegin(), s.rend(), not_space).base(), s.end());
+}
+
+
+static std::string
+default_config_path()
+{
+	const char *home = std::getenv("HOME");
+	if (!home || !*home)
+		return std::string();
+	std::string path(home);
+	path += "/.config/kte/kge.ini";
+	return path;
+}
+
+
+GUIConfig
+GUIConfig::Load()
+{
+	GUIConfig cfg; // defaults already set
+	std::string path = default_config_path();
+	if (!path.empty()) {
+		cfg.LoadFromFile(path);
+	}
+	return cfg;
+}
+
+
+bool
+GUIConfig::LoadFromFile(const std::string &path)
+{
+	std::ifstream in(path);
+	if (!in.good())
+		return false;
+
+	std::string line;
+	while (std::getline(in, line)) {
+		// Remove comments starting with '#' or ';'
+		auto hash = line.find('#');
+		if (hash != std::string::npos)
+			line.erase(hash);
+		auto sc = line.find("//");
+		if (sc != std::string::npos)
+			line.erase(sc);
+		// Basic key=value
+		auto eq = line.find('=');
+		if (eq == std::string::npos)
+			continue;
+		std::string key = line.substr(0, eq);
+		std::string val = line.substr(eq + 1);
+		trim(key);
+		trim(val);
+		// Strip trailing semicolon
+		if (!val.empty() && val.back() == ';') {
+			val.pop_back();
+			trim(val);
+		}
+
+		// Lowercase key
+		std::transform(key.begin(), key.end(), key.begin(), [](unsigned char c) {
+			return (char) std::tolower(c);
+		});
+
+		if (key == "fullscreen") {
+			fullscreen = (val == "1" || val == "true" || val == "on" || val == "yes");
+		} else if (key == "columns" || key == "cols") {
+			int v = columns;
+			try {
+				v = std::stoi(val);
+			} catch (...) {}
+			if (v > 0)
+				columns = v;
+		} else if (key == "rows") {
+			int v = rows;
+			try {
+				v = std::stoi(val);
+			} catch (...) {}
+			if (v > 0)
+				rows = v;
+		} else if (key == "font_size" || key == "fontsize") {
+			float v = font_size;
+			try {
+				v = std::stof(val);
+			} catch (...) {}
+			if (v > 0.0f)
+				font_size = v;
+		}
+	}
+
+	return true;
+}