Add Ventoy support and refactor wallpaper handling

Introduce a `ventoy.enable` option (default false) and import the new
`modules/core/ventoy.nix` in the core module list. All host
`system-modules.nix` files now disable Ventoy explicitly.

Make the boot wallpaper configurable by importing `stylixImage` from the
host’s `variables.nix` and using it in `modules/core/boot.nix`.

Update each host’s `variables.nix` to set a default wallpaper and keep
many alternatives commented out.

Remove the obsolete `restart.noctalia` and `screenshootin` scripts,
their Nix wrappers, and the related Hyprland keybinding.

Add a collection of new wallpaper files and delete the old, unused
images.

modules/home/scripts/default.nix

@@ -1,7 +1,5 @@
 {pkgs, ...}: {
   home.packages = [
     (import ./nvidia-offload.nix {inherit pkgs;})
-    (import ./restart.noctalia.nix {inherit pkgs;})
-    (import ./screenshootin.nix {inherit pkgs;})
   ];
 }