feat: add AirPlay playback backend

[zicochaos]

Summary

• Add a macOS-only AirPlay playback backend with Bonjour RAOP discovery.
• Stream decoded PCM through a separate AirPlay worker process using optional node-airtunes2.
• Add Settings support for selecting airplay, cycling AirPlay receivers with a, and submitting receiver codes via :airplay-code .
• Document macOS AirPlay setup and keep the AirPlay sender as an optional peer dependency so the default install/audit tree stays clean.

Test Plan

• npm run verify
• npm audit --audit-level=low
• Compiled AirPlay smoke test against Office Sonos receiver with optional peer supplied via NODE_PATH

Notes

node-airtunes2 is not a production dependency. AirPlay is advertised only when macOS has ffmpeg, dns-sd, and the optional sender package resolvable at runtime.

[vercel]

@zicochaos is attempting to deploy a commit to the Viewport Team on Vercel.

A member of the Team first needs to authorize it.