revert: restore original flat board and simple pawn visuals

Remove all 10 fancy visual enhancements (3D gradients, embossed borders,
glowing safe squares, specular pawns) — back to the clean flat style.
Keep 60fps render loop, smooth tweens, stacking, and mini dice.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>