Add unsigned macOS release automation

[Charlie284]

Summary

• package and retain an unsigned universal macOS build on every CI run
• publish checksum-backed GitHub prereleases when a v* tag is pushed
• inject the tag version into the app bundle and keep unsigned downloads clearly labeled
• document preview installation and the boundary between unsigned previews and notarized production releases

Testing

• reran the existing GitHub Actions workflow successfully after making the repository public
• validated both workflow files as YAML
• ran Swift formatting, property-list validation, shell syntax checks, and git diff --check
• simulated a 1.0.0 universal Release build with warnings as errors
• verified the unsigned app bundle, embedded version/build numbers, ZIP integrity, and SHA-256 generation