feat(games): implement chess game plugin with full UI

Add chess as the first game plugin using the existing multiplayer framework.
Server-side game logic uses chess.js with server-authoritative clock management.
Client uses react-chessboard v5 with cburnett piece set, drag-and-drop + click-to-move,
configurable time controls (bullet/blitz/rapid/classical/none), draw offers,
resignation, and timeout detection. Extends the game framework with room creation
options to support per-game configuration. Includes 57 tests covering all code paths.

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",