fix(overlay): patch font-manager 0.9.4 for gtk4 4.22 Vala errors

font-manager 0.9.4 fails to compile against gtk4 4.22: the
Gtk.DragIcon.get_for_drag binding changed in the gtk4 vapi, producing
'use new operator' errors in Collections.vala and FontList.vala that
broke home-manager-path for all desktop hosts.

0.9.4 is the latest upstream release (master unfixed too); the fix is
upstream PR FontManager/font-manager#468, already carried by nixpkgs
master but predating our locked nixpkgs and nixpkgs-master inputs.
Vendor the patch as an overlay mod, following the aardvark-dns pattern.
Drop once the nixpkgs lock carries it.

overlays/default.nix

@@ -7,10 +7,15 @@
   # This one contains whatever you want to overlay
   # You can change versions, add patches, set compilation flags, anything really.
   # https://nixos.wiki/wiki/Overlays
-  modifications = final: _prev: {
+  modifications = final: prev: {
     # aardvark-dns 2.x has a DNS forwarding bug breaking external name
     # resolution in podman networks — pin to last 1.x from nixpkgs-stable.
     aardvark-dns = import ./mods/aardvark-dns.nix {inherit (final) stable;};
+
+    # font-manager 0.9.4 does not compile against gtk4 4.22 (Vala errors
+    # in Gtk.DragIcon.get_for_drag) — apply the upstream fix from
+    # FontManager/font-manager#468 until our nixpkgs lock carries it.
+    font-manager = import ./mods/font-manager.nix {inherit prev;};
   };
 
   # temp-packages = final: _prev: {