Share the wallpaper symlink across all hosts by moving it from gnome.nix
into home-manager/fred.nix, and add matugen templates for btop and the
Homepage dashboard.
The Homepage NixOS module writes custom.css into /etc (read-only), so
bind-mount /var/lib/homepage-custom-css/custom.css over it. A systemd
path unit restarts homepage-dashboard whenever matugen rewrites the
file, so regeneration works without sudo.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
