Corrige widget de chat para nao quebrar sem API

- Verifica se api.liveChat.listAgentSessions existe antes de renderizar
- Retorna null no provider se Convex nao estiver sincronizado

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/components/chat/chat-widget.tsx

@@ -13,6 +13,7 @@ import { MessageCircle, Send, X, Minimize2, User, Headphones, ChevronDown } from
 
 const MAX_MESSAGE_LENGTH = 4000
 
+
 function formatTime(timestamp: number) {
   return new Date(timestamp).toLocaleTimeString("pt-BR", {
     hour: "2-digit",