fix: 修复文字对话态误显「正在初始化语音检测」

ChatPanel VAD 初始化提示条件增加 isCameraOn 判断，
视频结束后不再显示语音检测初始化提示。

frontend/src/App.tsx

@@ -520,6 +520,7 @@ function AppContent() {
               isProcessing={isProcessing}
               isVADReady={isVADReady}
               vadError={vadError ?? undefined}
+              isCameraOn={isCameraOn}
               isMicOn={isMicOn}
               isSpeaking={isSpeaking}
               onSendText={sendTextMessage}

frontend/src/components/ChatPanel/index.tsx

@@ -18,6 +18,7 @@ interface ChatPanelProps {
   isProcessing?: boolean;
   isVADReady?: boolean;
   vadError?: string | null;
+  isCameraOn?: boolean;
   isMicOn?: boolean;
   isSpeaking?: boolean;
   onSendText?: (text: string) => void;
@@ -44,6 +45,7 @@ export function ChatPanel({
   isProcessing,
   isVADReady,
   vadError,
+  isCameraOn,
   isMicOn,
   isSpeaking,
   onSendText,
@@ -198,7 +200,7 @@ export function ChatPanel({
         )}
 
         {/* VAD 初始化中 */}
-        {isConnected && isVADReady === false && !vadError && (
+        {isConnected && isCameraOn && isVADReady === false && !vadError && (
           <div className="system-message system-message--info">
             <span className="typing-indicator">
               <span className="typing-indicator__dot" />