Add reaction emoji overlay on board stones
When "All Reactions" mode is toggled, reaction emojis are overlaid
directly on the stones they reference, making it easy to see which
moves received reactions at a glance.
- Add reactionOverlay prop to Board component
- Compute overlay data mapping move reactions to stone coordinates
- Position emojis with drop shadow for visibility on both stone colors
- Show up to 3 unique emojis per stone, with +N count for more
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>