refactor: 删除实时分析/按需识别/纯聊天三种模式切换

三种模式仅为前端 UI 区分，后端无感知，实际使用中价值不大：
- 纯聊天模式名不副实（VAD 仍附带摄像头帧）
- 实时分析增加 API 成本且体验不佳
- 按需识别已是默认且最自然的交互方式

删除后保留按需识别行为：语音带图 + 手动识别按钮，UI 更简洁。

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

@@ -48,7 +48,6 @@ export const zhCN: TranslationMap = {
   "scenario.interpreter.hint": "AI 会实时翻译你的话（中英互译），无解释评论",
 
   // Video indicators
-  "video.observing": "👁️ 观察中",
   "video.listening": "🎤 正在聆听...",
   "video.playing": "🔊 正在播放...",
   "video.initVad": "正在初始化语音检测...",
@@ -67,8 +66,6 @@ export const zhCN: TranslationMap = {
   "controls.cameraOn": "开启摄像头",
   "controls.micOff": "关闭麦克风",
   "controls.micOn": "开启麦克风",
-  "controls.observing": "👁️ 观察中",
-  "controls.observation": "👁️ 观察模式",
   "controls.interrupt": "⏹ 打断",
   "controls.stop": "结束对话",
   "controls.stopVideo": "结束视频",
@@ -77,7 +74,6 @@ export const zhCN: TranslationMap = {
 
   // Chat panel
   "chat.title": "对话",
-  "chat.mode.observation": "观察模式",
   "chat.reconnecting": "连接已断开，正在重连...",
   "chat.connecting": "正在连接服务...",
   "chat.vadInit": "正在初始化语音检测...",
@@ -91,7 +87,6 @@ export const zhCN: TranslationMap = {
   "chat.scenarioSwitched": "已切换到 {name} 模式",
 
   // Session messages
-  "session.changeDetected": "👁️ 画面变化检测",
   "session.recognizing": "（语音识别中...）",
   "session.sttFailed": "（语音识别失败，请重试）",
   "session.noSpeech": "（未识别到语音）",
@@ -141,9 +136,6 @@ export const zhCN: TranslationMap = {
   "controls.device.camera": "摄像头",
   "controls.device.mic": "麦克风",
   "controls.device.default": "默认",
-  "controls.mode.realtime": "实时分析",
-  "controls.mode.ondemand": "按需识别",
-  "controls.mode.chat": "纯聊天",
 
   // Status bar
   "statusbar.ready": "就绪 · 选择设备后开始通话",