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.
