Live camera feed + recording for instructors

Let the instructor view the learner's device camera live during a session, with a separate one-click "Record" action to save video+audio to a folder on the instructor's own machine (not cloud storage). Requires camera/mic permission on the student's device. Since our controller and display already run on separate physical devices connected over the network, this needs genuine real-time video streaming between two devices (WebRTC — peer-to-peer video, typically needs a small signaling server, sometimes a relay server for restrictive networks). This is a multi-day engineering feature, not a UI toggle.