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/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",