Theme btop and Homepage via matugen on the mediaserver

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>

settings/gnome.nix

@@ -64,9 +64,6 @@
 
     # Home Manager GNOME settings
     home-manager.users.fred = { config, lib, ... }: {
-      home.file.".local/share/backgrounds/wallpaper.png".source =
-        "${inputs.self}/walls/wallpaper.png";
-
       # Minimal titlebars — hide window buttons and shrink headerbar
       home.file.".config/gtk-4.0/gtk.css".force = true;
       home.file.".config/gtk-4.0/gtk.css".text = ''