feat: recording playback speed control

## Summary

Add a `playbackSpeed` option to control the pace of GIF recordings (e.g., 1.5x, 2x). This would eliminate frames before deduplication to create faster-paced recordings.

## Use Case

Long recordings with lots of idle time can be compressed via frame deduplication, but the actual pace remains 1:1 with real-time. A playback speed option would allow creating snappier demos without manually editing.

## Proposed Approach

Add `playbackSpeed` parameter to `shell_record_stop`:
- `1.0` = normal speed (default)
- `1.5` = 50% faster
- `2.0` = double speed

Implementation: reduce frame delays by the speed factor before deduplication. For example, at 2x speed, a 100ms delay becomes 50ms.

## Example

```json
{
"session_id": "shell-session-abc",
"name": "demo",
"playbackSpeed": 1.5
}
```

## Notes

- Should be applied before frame deduplication for best results
- Could also be a render-time option in `renderGif()`