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.

hosts/default/system-modules.nix

@@ -36,6 +36,11 @@ _: {
   */
   affinity.enable = false; # Affinity: professional graphics suite
 
+  /*
+  Utilities / Misc
+  */
+  ventoy.enable = false; # Ventoy: New Bootable USB Solution with GUI support
+
   packages = {
     /*
     Container & Packaging