feat(panel): migrate to React Router, role-based layout and routing

Replace useState-based page switching with react-router-dom Routes.
Layout now renders admin or player nav items based on user.role.
Add stub pages for PlayerDashboard, GameLobby, and GameRoom.

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

panel/src/games/GameLobby.tsx

@@ -0,0 +1,3 @@
+export function GameLobby() {
+    return <div className="text-text-tertiary">Game Lobby — loading...</div>;
+}