fix(demo): stabilize video export

[ptj0225]

• ## Summary

• Stabilize video export from Exports > Video > Download Video.
• Pause playback while recording export frames.
• Flush each frame update before requesting the browser canvas render.
• Restore the original frame and playback state after export finishes.

Root Cause

The video export path iterates over every frame, calls demo.set_frame(...), and immediately requests a browser render with event_client.get_render(...).

That can race in two ways:

• If playback is active, the background playback loop can advance session.frame_idx while export is also stepping frames.
• Viser frame updates can remain in the websocket message buffer, so the render request may capture the previous or otherwise stale visual frame before the new frame is applied in the browser.

As a result, exported videos could appear to have frames out of order or visually mixed.

Fix

During video export:

• Save the current playback state.
• Set session.playing = False to stop the background playback loop from changing frames.
• Call event_client.flush() after each demo.set_frame(...) before get_render(...).
• Restore the original frame and flush it.
• Restore the original playback state in finally.