fix(proxy): stop screenshare when the connection stops

conference.js

                  */
                 convertVideoToDesktop: true,
 
+                /**
+                 * Callback invoked when the connection has been closed
+                 * automatically. Triggers cleanup of screensharing if active.
+                 *
+                 * @returns {void}
+                 */
+                onConnectionClosed: () => {
+                    if (this._untoggleScreenSharing) {
+                        this._untoggleScreenSharing();
+                    }
+                },
+
                 /**
                  * Callback invoked to pass messages from the local client back
                  * out to the external client.