OptionalonOpen the recording-settings dialog (mic / camera selection).
scheduledStart: optional performance.now() timestamp at which
playback should be considered position 0. The toolbar's Play All
always omits it (composition picks a default lookahead); the
recorder passes a value to align Play All with its own
MediaRecorder.start() so the new recording's t=0 lines up with
the existing clips' t=0 within JS scheduler tolerance.
Props for ClipToolbar.