Hotspot forward navigation button on voiceover completion

Add an option to hide (or disable) the forward hotspot navigation button until a step's voiceover has finished playing, so viewers can only advance once the current step is fully explained.

Some steps have longer explanations that are essential to understanding the demo. Right now viewers can click forward immediately and skip past the audio, missing key context. Gating the forward button on VO completion ensures the explanation is actually heard, not skipped.

Suggested behavior:

• A per-demo or per-step toggle (e.g., "Require voiceover completion before advancing")

• While VO plays, the forward button is hidden or greyed out; once it ends, the button appears/enables normally

• Back navigation stays available at any time — only the forward action is gated