docs: add application screenshot

README.md

@@ -2,6 +2,8 @@
 
 A local, responsive web interface for Sonos with automatic device discovery through [SoCo](https://github.com/SoCo/SoCo). It controls playback and volume, manages groups, and plays Sonos favorites, items from the current queue, or direct HTTP(S) streams. The application stores no state and does not require a database.
 
+![Sonos NestControl dashboard](assets/sonos-web-screenshot.png)
+
 ## Security boundary
 
 **This application intentionally has no authentication. Run it exclusively on a trusted LAN and never expose it to the internet through port forwarding, a reverse proxy, or a public IP address.** Any client with access to port 5000 can control every discovered Sonos device. Firewall rules should restrict access to the local management network.