add overlay to fix firebase-tools build error

overlays/firebase-tools.nix

@@ -0,0 +1,17 @@
+# Standalone overlay for firebase-tools
+# This file exports a single overlay function that forces `firebase-tools`
+# to use `nodejs_22` when available, falling back to `nodejs` otherwise.
+#
+# It's intended to be imported directly in a host's `nixpkgs.overlays`,
+# e.g. in `hosts/server/configuration.nix`.
+final: prev:
+let
+  # Prefer nodejs_22 if present in the previous package set; otherwise use nodejs.
+  nodejsChoice = if builtins.hasAttr "nodejs_22" prev then prev.nodejs_22 else prev.nodejs;
+in
+{
+  # Override firebase-tools to use the chosen Node.js package.
+  firebase-tools = prev.firebase-tools.override {
+    nodejs = nodejsChoice;
+  };
+}