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/network.nix

@@ -34,6 +34,8 @@
         }
       ];
       allowedUDPPorts = [
+        5353
+        54925
         59010
         59011
       ];