fix: 修复 ChatPanel vadError 属性类型不匹配导致构建失败

frontend/src/components/ChatPanel/index.tsx

@@ -15,7 +15,7 @@ interface ChatPanelProps {
   connectionStatus: ConnectionStatus;
   isProcessing?: boolean;
   isVADReady?: boolean;
-  vadError?: string;
+  vadError?: string | null;
   isMicOn?: boolean;
   isSpeaking?: boolean;
   onSendText?: (text: string) => void;