feat: validate desktop environment

Check that the desktop environment currently being used is GNOME.

usr/local/bin/wallpaperctl

@@ -3,6 +3,10 @@ gversion="0"
 pictureuri="picture-uri"
 
 validate_config() {
+    if [ "$XDG_CURRENT_DESKTOP" != "GNOME" ]; then
+        quit "Error: GNOME is not detected. Please install GNOME."
+    fi
+
     if [ -z "$WALLPAPER_DIR" ]; then
         quit "Error: The wallpaper directory environment variable (WALLPAPER_DIR) has not been set."
     fi