fix: register ludo unmountGame to clean up timers on scene exit

Without this, the render loop, polling, heartbeat, and turn timers
would continue running after navigating away from the game scene.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/js/modules/ludo/mod.js

 import * as scene from '../../core/scene.js';
-import { mountGame } from './scenes/game.js';
+import { mountGame, unmountGame } from './scenes/game.js';
 import { mountResult } from './scenes/result.js';
 import { mountRoom, unmountRoom } from './scenes/room.js';
 
-scene.register('ludo-game', mountGame);
+scene.register('ludo-game', mountGame, unmountGame);
 scene.register('ludo-result', mountResult);
 scene.register('ludo-room', mountRoom, unmountRoom);