Consolidate printer config and enable simple-scan

Remove per‑host printers.nix and merge printing settings into host
variables.
Add simple-scan package and enable it for desktop and laptop hosts.
Disable simple-scan for the default host.
Enable SANE daemon and expose its libraries in fish init.
Open required UDP ports 5353 and 54925 in network module.

hosts/default/variables.nix

@@ -68,4 +68,22 @@
   # Set network hostId if required (needed for zfs)
   # Otherwise leave as-is
   hostId = "5ab03f50";
+
+  # 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";
+    }
+  ];
 }