fix: guard against undefined username in trivia won event

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

shared/lib/eventWiring.ts

@@ -74,7 +74,7 @@ export function registerDomainEventListeners() {
         fireAndForget(async () => {
             await dashboardService.recordEvent({
                 type: 'success',
-                message: `${payload.username} won ${payload.reward.toLocaleString()} AU from trivia!`,
+                message: `${payload.username ?? 'Unknown user'} won ${payload.reward.toLocaleString()} AU from trivia!`,
                 icon: '🎉'
             });
         });