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/boot.nix

@@ -1,9 +1,12 @@
 {
   pkgs,
+  host,
   config,
   lib,
   ...
-}: {
+}: let
+  inherit (import ../../hosts/${host}/variables.nix) stylixImage;
+in {
   boot = {
     kernelPackages = pkgs.linuxPackages_zen;
     kernelModules = ["v4l2loopback"];
@@ -17,7 +20,7 @@
         enable = true;
         style = {
           wallpapers = [
-            ./../../wallpapers/attack-on-titan-mikasa-ackerman.jpg
+            ./../../wallpapers/${stylixImage}
           ];
           wallpaperStyle = lib.mkForce "centered";
           backdrop = "${config.stylix.base16Scheme.base00}";