feat(qbzd): scaffold headless daemon binary (Phase 0)

New workspace member crates/qbzd/ with:
- CLI interface (clap): port, bind, data-dir, config, token, log-level
- Config system (TOML): server, audio, cache, data, mpris, qconnect, mdns
- Resource detection (sysinfo): auto-size cache from available RAM
- DaemonAdapter: FrontendAdapter impl routing CoreEvents to broadcast channel
- DaemonCore: state container replacing Tauri's app.state
- Minimal HTTP server (Axum): /api/ping + /api/info
- Graceful shutdown on SIGTERM/SIGINT

Zero Tauri dependency. Desktop app unaffected.

cc

crates/Cargo.toml

@@ -16,6 +16,7 @@ members = [
     "qconnect-protocol",
     "qconnect-transport-ws",
     "qconnect-app",
+    "qbzd",
 ]
 
 [workspace.package]