Enable sane-airscan and wrap Simple Scan

Add avahi, ipp-usb, udev (with sane‑airscan) and saned support.
Configure `sane` to enable extra backends (sane‑airscan) and disable the
default ESCL backend.
Override `simple-scan` to wrap its binary with an LD_LIBRARY_PATH that
includes `/etc/sane-libs`.

modules/core/packages/packages.nix

@@ -76,7 +76,14 @@
     notify = notify-client;
     boxes = gnome-boxes;
     switcheroo = switcheroo;
-    simpleScan = simple-scan;
+    simpleScan = simple-scan.overrideAttrs (old: {
+      postFixup =
+        (old.postFixup or "")
+        + ''
+          wrapProgram $out/bin/simple-scan \
+            --prefix LD_LIBRARY_PATH : "/etc/sane-libs"
+        '';
+    });
   };
 in {
   imports = builtins.attrValues (builtins.mapAttrs mkPackage packages);