flake: add cachix binary cache

Adds nixConfig substituter/key for the invoiceninja-nixos cachix cache,
a `just push-cache` target to build and push the package and automated
test to it, and exposes the UI frontend build as a passthru output
(`invoiceninja.ui`) so it can be built and pushed independently.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

flake.nix

@@ -1,6 +1,13 @@
 {
   description = "An Invoice Ninja package and module.";
 
+  nixConfig = {
+    extra-substituters = [ "https://invoiceninja-nixos.cachix.org" ];
+    extra-trusted-public-keys = [
+      "invoiceninja-nixos.cachix.org-1:TmRZaji8FFVjjIw9KZmvLrxMxfP+JDThOyOX04z526A="
+    ];
+  };
+
   inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-26.05";
 
   outputs =