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>

justfile

@@ -36,3 +36,11 @@ boot-vm:
 test:
     nix flake check -L
 
+[doc('Build the package and automated test, then push results to the invoiceninja-nixos cachix cache')]
+[group('maintenance')]
+push-cache:
+    nix build --print-out-paths --no-link \
+        ".#invoiceninja" \
+        ".#checks.x86_64-linux.invoiceninja" \
+        | cachix push invoiceninja-nixos
+