Add "Show my camera": a local, mirrored self-view on the interview stage

Off by default. The camera is asked for only when turned on, as its own video-only stream, so a refusal or a missing camera shows a short message and never touches the microphone or the interview. The self-view sits in the stage's bottom corner, clear of the avatar's face and the captions, in both the video interview and the phone screen. Nothing is recorded, saved or sent; the tracks stop when it is turned off and when the interview finishes.