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/mods/font-manager.nix

@@ -0,0 +1,19 @@
+# font-manager 0.9.4: fix compilation against gtk4 >= 4.22 / newer Vala.
+#
+# In GTK 4.22 the Gtk.DragIcon.get_for_drag binding changed in the gtk4
+# vapi, turning `Gtk.DragIcon.get_for_drag(drag)` into a hard Vala error
+# ("use `new' operator to create new objects") in Collections.vala and
+# FontList.vala.  0.9.4 is the latest upstream release and even upstream
+# master is unfixed; the fix lives in FontManager/font-manager#468 and is
+# already carried as a patch by current nixpkgs master — but our locked
+# nixpkgs (and the locked nixpkgs-master input) predate it.
+#
+# Drop this overlay (and the patch file) once our nixpkgs lock advances
+# past nixpkgs@e2ad529-vendoring (pkgs/by-name/fo/font-manager carries
+# fix-compilation-error-with-newer-vala-versions.patch).
+{prev}:
+prev.font-manager.overrideAttrs (old: {
+  patches =
+    (old.patches or [])
+    ++ [./patches/font-manager-fix-compilation-error-with-newer-vala-versions.patch];
+})