Coordinates now inside squares with contrasting colors:
- Rank numbers: top-left of left column, alternating light/dark
- File letters: bottom-right of bottom row, alternating dark/light
- No separate border/padding needed
- Board: no thick border, just subtle shadow
- Clean minimal look matching Chess.com mobile
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>