Daemon side: registers _qbz._tcp.local mDNS service on startup
so desktop apps can discover it on the LAN. Uses hostname as
service name (configurable via qbzd.toml).
Desktop side: new qbzd_discovery.rs module with Tauri commands:
- v2_qbzd_start_discovery: browse _qbz._tcp services
- v2_qbzd_stop_discovery: stop browsing
- v2_qbzd_get_devices: list discovered daemons
Uses same mdns-sd crate already used for AirPlay discovery.
cc
vicrodh
8fc6111782