refactor: remove embedded wallpaper from binary (v0.5.2)

Wallpaper is installed by moonarch to /usr/share/moonarch/wallpaper.jpg.
Embedding a 374K JPEG in the binary was redundant. Without a wallpaper
file, GTK background color (Catppuccin Mocha base) shows through and
wallpaper-only windows on secondary monitors are skipped.

Cargo.lock

@@ -569,7 +569,7 @@ dependencies = [
 
 [[package]]
 name = "moongreet"
-version = "0.5.0"
+version = "0.5.2"
 dependencies = [
  "gdk-pixbuf",
  "gdk4",