ci: add end-to-end integration workflow with zwave-js-ui #240

We currently lack a real-world integration test that validates pkg against a large, representative Node.js project. zwave-js-ui is the ideal candidate — it's complex, actively maintained, and a known consumer of pkg.

Proposal

Create a GitHub Actions workflow that:

1. Clones the zwave-js-ui repository
2. Packages it using both standard and SEA modes
3. Runs the produced binaries with basic smoke tests (e.g. startup, health endpoint, graceful shutdown)
4. Reports binary size, build time, and startup time

This would catch regressions early and give us confidence when landing changes that touch the packer, producer, or SEA pipelines.

Part of #235