feat(nixos): add Bambu Studio Flatpak

Manage the user Flatpak through nix-flatpak and enable printer discovery through the host firewall.

flake.nix

@@ -16,6 +16,9 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
 
+    # Installs Flatpak applications declaratively.
+    nix-flatpak.url = "github:gmodena/nix-flatpak/?ref=v0.7.0";
+
     # Provides signed AMO extensions pinned by version and hash.
     firefox-addons = {
       url = "gitlab:rycee/nur-expressions?dir=pkgs/firefox-addons";