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>

package.nix

@@ -74,6 +74,8 @@ php82.buildComposerProject (finalAttrs: {
   inherit src version;
   pname = "${pname}-composer";
 
+  passthru.ui = ui;
+
   nativeBuildInputs = [
     jq
     nodejs_22