Fix session cleanup: send endSession when stopping stream

The client now notifies the server when a session ends, preventing
stale session mappings that cause connection issues after multiple
start/stop cycles.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

index.html

@@ -865,9 +865,15 @@
             }, 2000); // Check every 2 seconds
         }
 
-        function stopStream() {
+        async function stopStream() {
             if (latencyMonitorId) clearInterval(latencyMonitorId);
             if (stateRefreshInterval) clearInterval(stateRefreshInterval);
+
+            // Notify server that session is ending
+            if (currentSessionId) {
+                await sendToServer({ type: 'endSession', sessionId: currentSessionId });
+            }
+
             if (peerConnection) peerConnection.close();
             if (dataChannel) dataChannel.close();
             peerConnection = null;