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/printing.nix

@@ -31,8 +31,12 @@
     ipp-usb.enable = true;
   };
 
-  hardware.printers = lib.mkIf printEnable {
-    ensureDefaultPrinter = ensureDefaultPrinter;
-    ensurePrinters = ensurePrinters;
+  hardware = lib.mkIf printEnable {
+    printers = {
+      ensureDefaultPrinter = ensureDefaultPrinter;
+      ensurePrinters = ensurePrinters;
+    };
+
+    sane.enable = true;
   };
 }