Add a GitHub Actions workflow that validates the app on pushes and pull requests, and publishes Windows, macOS, and Linux release assets when a tag is pushed.
Also replace the Windows-only packaging flow with cross-platform Bun scripts for sidecar publishing and Electron packaging so the release job can package each runner natively.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
