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.

hosts/default/printers.nix

@@ -0,0 +1,21 @@
+{
+  # Whether printing services should be enabled.
+  printEnable = false;
+
+  # Name of the default printer.
+  ensureDefaultPrinter = "[name]";
+
+  # List of printers to ensure are configured.
+  ensurePrinters = [
+    {
+      # URI of the printer device.
+      deviceUri = "ipp://[IPv4]/ipp";
+      # Physical location description of the printer.
+      location = "[location]";
+      # Human‑readable printer name.
+      name = "[name]";
+      # Printer model; "everywhere" uses generic driver.
+      model = "everywhere";
+    }
+  ];
+}