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.

modules/core/packages/packages.nix

@@ -76,6 +76,7 @@
     notify = notify-client;
     boxes = gnome-boxes;
     switcheroo = switcheroo;
+    simpleScan = simple-scan;
   };
 in {
   imports = builtins.attrValues (builtins.mapAttrs mkPackage packages);