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>

panel/package.json

@@ -10,10 +10,12 @@
   },
   "dependencies": {
     "@imgly/background-removal": "^1.7.0",
+    "chess.js": "^1.4.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "lucide-react": "^0.564.0",
     "react": "^19.1.0",
+    "react-chessboard": "^5.10.0",
     "react-dom": "^19.1.0",
     "react-router-dom": "^7.13.2",
     "tailwind-merge": "^3.4.0",