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",

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",