Converted all hardcoded chess board values to CSS custom properties:
- Board: radius, shadow, border, max-width
- Pieces: size, white/black colors, stroke, shadow, hover/drag scale
- Legal moves: dot size, capture ring size/border
- Clock: bg, active bg, radius, font-size, padding
- Coordinates: size, weight
- Side panel: width, bg, radius
- Move list: font-size
- Eval bar: width, radius
- Game header: padding, radius, bg, player/rating font sizes

All 32 variables exposed in admin theme under "♟ Chess Styling" section
with text inputs for full customization of every single value.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>