feat(client): implement server-initiated mode with WebSocket support

contrib/systemd/sendspin-client.service

@@ -4,8 +4,10 @@ After=pipewire.service network-online.target
 Wants=pipewire.service
 
 [Service]
+# No --server: server-initiated mode. The player advertises via mDNS and
+# waits for Music Assistant to discover and connect to it. Pass --server
+# host:port only to force the old client-initiated (dial-out) behavior.
 ExecStart=%h/.local/bin/sendspin-client \
-    --server 192.168.1.100:8927 \
     --name %H \
     --device alsa_output.usb-Device-00.analog-stereo \
     --volume 80