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 AZ-LT-NIX.

Fix is upstream PR FontManager/font-manager#468, already carried by
nixpkgs master but predating our locked nixpkgs-unstable. Vendor the
patch as an overlay mod (from m3tam3re/nixos-config overlays/mods) and
enable outputs.overlays.modifications in hosts/common. Thin clients are
unaffected - they do not import hosts/common. Drop once the nixpkgs
lock carries fix-compilation-error-with-newer-vala-versions.patch.

Also resolve the evaluation warning on AZ-LT-NIX where
programs.nh.clean.enable and nix.gc.automatic were both enabled: keep
the nh clean timer (--keep 3 --keep-since 4d) and disable the plain
nix-gc timer via mkForce on this host only.

hosts/common/default.nix

@@ -31,7 +31,7 @@
     overlays = [
       # Add overlays your own flake exports (from overlays and pkgs dir):
       #outputs.overlays.additions
-      #outputs.overlays.modifications
+      outputs.overlays.modifications
       outputs.overlays.stable-packages
       # outputs.overlays.pinned-packages