fix: migrate to node-pty@1.1.0-beta39 for Electron 38 support (#660)

- Replace @homebridge/node-pty-prebuilt-multiarch with official node-pty
beta
- Move electron to optionalDependencies to fix 'bunx mux@next' server
mode
- Add conditional postinstall script that rebuilds only when Electron
present
- Update electron-builder asarUnpack to include node-pty native binaries
- Improve PTYService error messages for desktop vs server mode failures

Fixes terminal issues in both desktop (Electron 38) and server modes.
Desktop mode: automatic rebuild for Electron ABI on install Server mode:
uses Node.js prebuilt binaries (no rebuild needed)

Author: sreya