feat: 结束视频后保留对话，支持继续文字聊天

- useVisionSession 新增 stopVideo 回调（停媒体流，保持连接和消息）
- App.tsx 控制区从二态改为三态（initial/video/textOnly）
- 文字对话态显示「重新开始视频」和「结束会话」按钮
- 新增 CSS 样式（video-ended-hint、btn--outline）
- 新增 i18n key（stopVideo/endSession/resumeVideo/video.ended）
- 新增设计方案文档

frontend/src/App.css

@@ -1503,6 +1503,48 @@ body {
   color: white;
 }
 
+/* ---- Text-only mode (video ended, chat preserved) ---- */
+
+.video-controls__text-only {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 10px;
+  width: 100%;
+}
+
+.video-ended-hint {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 2px;
+  padding: 4px 0;
+}
+
+.video-ended-hint__title {
+  font-size: 0.85rem;
+  font-weight: 600;
+  color: var(--color-text-muted);
+}
+
+.video-ended-hint__sub {
+  font-size: 0.7rem;
+  color: var(--color-text-muted);
+  opacity: 0.7;
+}
+
+.btn--outline {
+  background: transparent;
+  border: 1px solid var(--color-border);
+  color: var(--color-text-muted);
+}
+
+.btn--outline:hover {
+  background: var(--color-surface-2);
+  color: var(--color-text);
+  border-color: var(--color-surface-3);
+}
+
 /* ---- Scene Cards (empty state) ---- */
 
 .scene-cards {

frontend/src/App.tsx

@@ -80,6 +80,7 @@ function AppContent() {
     isObserving,
     startSession,
     stopSession,
+    stopVideo,
     interrupt,
     isCameraOn,
     isMicOn,
@@ -397,9 +398,9 @@ function AppContent() {
                   </button>
                 </div>
               </>
-            ) : (
+            ) : isCameraOn ? (
               <>
-                {/* 通话态：核心控制工具栏 */}
+                {/* 视频通话态：核心控制工具栏 */}
                 <div className="video-controls__toolbar">
                   <button
                     className={`btn btn--ctrl ${isCameraOn ? "btn--ctrl-on" : "btn--ctrl-off"}`}
@@ -430,8 +431,8 @@ function AppContent() {
                       {tr("controls.interrupt")}
                     </button>
                   )}
-                  <button className="btn btn--danger" onClick={stopSession}>
-                    {tr("controls.stop")}
+                  <button className="btn btn--danger" onClick={stopVideo}>
+                    {tr("controls.stopVideo")}
                   </button>
                 </div>
                 {/* 通话态模式切换 */}
@@ -456,6 +457,24 @@ function AppContent() {
                   </button>
                 </div>
               </>
+            ) : (
+              <>
+                {/* 文字对话态：视频已结束 */}
+                <div className="video-controls__text-only">
+                  <div className="video-ended-hint">
+                    <span className="video-ended-hint__title">📹 {tr("video.ended")}</span>
+                    <span className="video-ended-hint__sub">{tr("video.ended.hint")}</span>
+                  </div>
+                  <div className="video-controls__toolbar">
+                    <button className="btn btn--primary" onClick={startSession}>
+                      {tr("controls.resumeVideo")}
+                    </button>
+                    <button className="btn btn--danger btn--outline" onClick={stopSession}>
+                      {tr("controls.endSession")}
+                    </button>
+                  </div>
+                </div>
+              </>
             )}
           </div>
         </div>

frontend/src/hooks/useVisionSession.ts

@@ -363,6 +363,22 @@ export function useVisionSession(accessToken?: string | null) {
     setIsMicOn(false);
   }, [stopObserving, stopVAD, stopMic, stopCamera, disconnect]);
 
+  /** 结束视频，保留聊天和连接 */
+  const stopVideo = useCallback(async () => {
+    stopObserving();
+    setMode("dialogue");
+    await stopVAD();
+    stopMic();
+    stopCamera();
+    ttsPlayerRef.current?.stop();
+    setIsAudioPlaying(false);
+    setCurrentReply("");
+    setIsProcessing(false);
+    setIsCameraOn(false);
+    setIsMicOn(false);
+    // 不断开 WebSocket，不清空消息、历史、统计
+  }, [stopObserving, stopVAD, stopMic, stopCamera]);
+
   /** 摄像头开关 */
   const toggleCamera = useCallback(async () => {
     if (isCameraOn) {
@@ -479,6 +495,7 @@ export function useVisionSession(accessToken?: string | null) {
     toggleMode,
     startSession,
     stopSession,
+    stopVideo,
     interrupt,
     isCameraOn,
     isMicOn,

frontend/src/lib/i18n/en-US.ts

@@ -51,6 +51,8 @@ export const enUS: TranslationMap = {
   "video.placeholder": "Type in the chat panel to start",
   "video.cameraOff": "Camera is off",
   "video.cameraOff.hint": "You can type in the chat panel",
+  "video.ended": "Video Ended",
+  "video.ended.hint": "You can continue chatting below",
 
   // Controls
   "controls.connecting": "Connecting...",
@@ -64,6 +66,9 @@ export const enUS: TranslationMap = {
   "controls.observation": "👁️ Observe",
   "controls.interrupt": "⏹ Interrupt",
   "controls.stop": "End Session",
+  "controls.stopVideo": "End Video",
+  "controls.endSession": "End Session",
+  "controls.resumeVideo": "📹 Resume Video",
 
   // Chat panel
   "chat.title": "Chat",

frontend/src/lib/i18n/ja-JP.ts

@@ -51,6 +51,8 @@ export const jaJP: TranslationMap = {
   "video.placeholder": "右側のチャットに入力して開始",
   "video.cameraOff": "カメラがオフです",
   "video.cameraOff.hint": "右側のチャットでテキスト対話ができます",
+  "video.ended": "ビデオ終了",
+  "video.ended.hint": "下にテキストを入力して会話を続けることができます",
 
   // Controls
   "controls.connecting": "接続中...",
@@ -64,6 +66,9 @@ export const jaJP: TranslationMap = {
   "controls.observation": "👁️ 観察モード",
   "controls.interrupt": "⏹ 中断",
   "controls.stop": "対話を終了",
+  "controls.stopVideo": "ビデオ終了",
+  "controls.endSession": "セッション終了",
+  "controls.resumeVideo": "📹 ビデオ再開",
 
   // Chat panel
   "chat.title": "チャット",

frontend/src/lib/i18n/zh-CN.ts

@@ -51,6 +51,8 @@ export const zhCN: TranslationMap = {
   "video.placeholder": "在右侧聊天框输入即可开始对话",
   "video.cameraOff": "摄像头未开启",
   "video.cameraOff.hint": "可在右侧聊天框打字对话",
+  "video.ended": "视频已结束",
+  "video.ended.hint": "您可以继续在下方输入文字对话",
 
   // Controls
   "controls.connecting": "连接中...",
@@ -64,6 +66,9 @@ export const zhCN: TranslationMap = {
   "controls.observation": "👁️ 观察模式",
   "controls.interrupt": "⏹ 打断",
   "controls.stop": "结束对话",
+  "controls.stopVideo": "结束视频",
+  "controls.endSession": "结束会话",
+  "controls.resumeVideo": "📹 重新开始视频",
 
   // Chat panel
   "chat.title": "对话",