Add host printer configs and refactor printing

Add printers.nix for Garand‑Desktop, Garand‑Laptop, and default hosts
with
printer definitions and a printEnable flag. Disable NFS by default in
the
host variables. Rewrite modules/core/printing.nix to import printer
settings, enable services only when printing is enabled, and add more
CUPS/Avahi options plus hardware.printers configuration. Add the lpadmin
group to the main user. Update the Messenger desktop entry URL to the
Facebook Messages page.

modules/core/user.nix

@@ -51,6 +51,7 @@ in {
         "wheel" # sudo access
         "vboxusers" # Virtual Box
         "gamemode"
+        "lpadmin"
       ];
       shell = pkgs.bash;
       ignoreShellProgramCheck = true;