feat(qbzd): mDNS discovery + registration for LAN pairing

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

src-tauri/Cargo.toml

@@ -28,6 +28,7 @@ qbz-cache = { path = "../crates/qbz-cache" }
 qbz-library = { path = "../crates/qbz-library" }
 qbz-cast = { path = "../crates/qbz-cast" }
 qbz-cmaf = { path = "../crates/qbz-cmaf" }
+mdns-sd = "0.11"
 qconnect-app = { path = "../crates/qconnect-app" }
 qconnect-core = { path = "../crates/qconnect-core" }
 qconnect-transport-ws = { path = "../crates/qconnect-transport-ws" }