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/public/pieces/bP.svg

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