Update MobileCallPage and useVoicePreview to handle call end state

- Introduce a new state variable `callEnded` in `useVoicePreview` to manage call termination more effectively.
- Modify `MobileCallPage` to adjust the `inCall` condition based on `callEnded`, ensuring accurate UI updates during and after calls.
- Update rendering logic to display appropriate messages based on call status, enhancing user feedback during call transitions.
- Refactor button visibility to only show when in an active call, improving user interface clarity.

frontend/src/hooks/use-voice-preview.ts

@@ -214,6 +214,9 @@ export function useVoicePreview(
   // 远端(助手 TTS)媒体流:除挂到 <audio> 播放外,也暴露给波形可视化
   const [remoteStream, setRemoteStream] = useState<MediaStream | null>(null);
   const [messages, setMessages] = useState<ChatMessage[]>([]);
+  // 后端结束节点/EndFrame 触发的正常结束。与 failed 区分，供独立通话页
+  // 立即收起媒体画面并展示重新开始入口。
+  const [callEnded, setCallEnded] = useState(false);
   const [networkQuality, setNetworkQuality] =
     useState<NetworkQuality>("unknown");
   // 可选麦克风/扬声器列表与当前选择(空串表示交给浏览器选默认设备)
@@ -398,6 +401,7 @@ export function useVoicePreview(
     setError(null);
     setMicWarning(null);
     setMessages([]); // 新会话清空上一轮聊天记录
+    setCallEnded(false);
     endedByServerRef.current = false;
     setStatus("connecting");
 
@@ -630,9 +634,12 @@ export function useVoicePreview(
             // 工作流:后端报告当前激活节点,交给画布高亮
             onNodeActiveRef.current?.(msg.nodeId);
           } else if (msg?.type === "call-ended") {
-            // 后端走到结束节点、正常收尾:随后的断开按正常结束处理,不报错
+            // 后端已播完结束节点的收尾语，即将排入 EndFrame。此时主动释放
+            // 浏览器侧资源，不依赖后续 WebRTC 状态事件是否及时到达。
             endedByServerRef.current = true;
+            setCallEnded(true);
             onNodeActiveRef.current?.(null);
+            disconnect();
           }
         } catch {
           /* 非 JSON / 未知消息,忽略 */
@@ -689,7 +696,14 @@ export function useVoicePreview(
     } finally {
       startingRef.current = false;
     }
-  }, [assistantId, applyOutputDevice, fail, closeOnRemoteEnd, refreshDevices]);
+  }, [
+    assistantId,
+    applyOutputDevice,
+    fail,
+    closeOnRemoteEnd,
+    disconnect,
+    refreshDevices,
+  ]);
 
   useEffect(() => {
     if (status !== "connected") return;
@@ -826,6 +840,7 @@ export function useVoicePreview(
     localStream,
     remoteStream,
     messages,
+    callEnded,
     networkQuality,
     audioInputs,
     audioOutputs,