feat(packages): add nftablesbuilder package

Web interface to manage nftables rules with drag-and-drop rule creation.

Upstream source is incomplete (missing settings crate, no GUI build
tooling), so the package builds the Rust workspace from the pinned
commit with a patched-in settings crate (schema recovered from official
release artifacts) and reuses the prebuilt GUI from the hash-pinned
release tarball on nftablesbuilder.eu.

packages/nftablesbuilder/default.nix

@@ -0,0 +1,5 @@
+{
+  pkgs,
+  ...
+}:
+pkgs.callPackage ./package.nix { }