Fix bidirectional audio: use sendrecv on existing transceiver

Instead of adding a new sendonly transceiver (which creates a new m-line
the server may not handle), find the existing audio transceiver from the
robot's stream and change its direction to sendrecv. This allows both
receiving robot audio AND sending mic audio on the same transceiver.

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

index.html

@@ -930,8 +930,25 @@
                 if (msg.sdp) {
                     await peerConnection.setRemoteDescription(new RTCSessionDescription(msg.sdp));
                     if (msg.sdp.type === 'offer') {
-                        const transceiver = peerConnection.addTransceiver('audio', { direction: 'sendonly' });
-                        audioSender = transceiver.sender;
+                        // Find the audio transceiver and change direction to sendrecv
+                        // so we can both receive robot audio AND send our mic audio
+                        const transceivers = peerConnection.getTransceivers();
+                        for (const transceiver of transceivers) {
+                            if (transceiver.receiver.track?.kind === 'audio') {
+                                transceiver.direction = 'sendrecv';
+                                audioSender = transceiver.sender;
+                                console.log('Audio transceiver set to sendrecv');
+                                break;
+                            }
+                        }
+
+                        // If no audio transceiver found, add one
+                        if (!audioSender) {
+                            const transceiver = peerConnection.addTransceiver('audio', { direction: 'sendrecv' });
+                            audioSender = transceiver.sender;
+                            console.log('Added new audio transceiver');
+                        }
+
                         const answer = await peerConnection.createAnswer();
                         await peerConnection.setLocalDescription(answer);
                         await sendToServer({ type: 'peer', sessionId: currentSessionId, sdp: { type: 'answer', sdp: answer.sdp } });