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/AZ-LT-NIX/programs.nix

@@ -1,4 +1,8 @@
-{pkgs, ...}: {
+{
+  lib,
+  pkgs,
+  ...
+}: {
   programs.nix-ld.enable = true;
   programs.nix-ld.libraries = with pkgs; [
     # Add any missing dynamic libraries for unpackaged programs
@@ -38,6 +42,10 @@
     clean.extraArgs = "--keep-since 4d --keep 3";
     flake = "/home/m3tam3re/p/nixos/nixos-config";
   };
+  # nh.clean and nix.gc.automatic conflict (NixOS assertion) — this host
+  # uses the nh clean timer above for generation/GC retention, so disable
+  # the plain nix-gc timer inherited from hosts/common.
+  nix.gc.automatic = lib.mkForce false;
   services.netbird.enable = true;
   environment.systemPackages = [pkgs.netbird-ui];
 }

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