feat(chess): replace custom engine with chess.js and react-chessboard

Swap the custom move validation and Unicode piece grid for chess.js
(full rules engine with check/checkmate/castling/en passant/promotion)
and react-chessboard (drag-and-drop SVG board). Board styled to match
the purple dark theme and auto-orients to the player's color.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -40,6 +40,7 @@
   },
   "dependencies": {
     "@napi-rs/canvas": "^0.1.89",
+    "chess.js": "^1.4.0",
     "discord.js": "^14.25.1",
     "dotenv": "^17.2.3",
     "drizzle-orm": "^0.44.7",