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/core/ventoy.nix

@@ -0,0 +1,15 @@
+{
+  pkgs,
+  lib,
+  config,
+  ...
+}: {
+  options.ventoy.enable = lib.mkEnableOption "Enable Ventoy";
+
+  config = lib.mkIf config.ventoy.enable {
+    environment.systemPackages = with pkgs; [ventoy-full-gtk];
+    nixpkgs.config.permittedInsecurePackages = [
+      "ventoy-gtk3-1.1.12"
+    ];
+  };
+}