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>

hosts/FredOS-Mediaserver.nix

@@ -17,6 +17,7 @@
       screen
       yt-dlp
       ghostty.terminfo
+      matugen
       (pkgs.writeShellScriptBin "transcode-hevc" ''
         export PATH="${pkgs.jellyfin-ffmpeg}/bin:${pkgs.coreutils}/bin:${pkgs.findutils}/bin:${pkgs.gnugrep}/bin:${pkgs.gawk}/bin:${pkgs.bc}/bin:${pkgs.curl}/bin:$PATH"
         exec ${pkgs.bash}/bin/bash ${../scripts/transcode-hevc.sh} "$@"