feat：语音对话优化

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

@@ -54,6 +54,8 @@ export const enUS: TranslationMap = {
   "chat.title": "Chat",
   "chat.mode.observation": "Observing",
   "chat.reconnecting": "Connection lost, reconnecting...",
+  "chat.connecting": "Connecting to server...",
+  "chat.vadInit": "Initializing voice detection...",
   "chat.empty.prompt": "Type below to start chatting",
   "chat.empty.hint": "Type to chat with AI, or click the button on the left to start video",
   "chat.welcome.prompt": "Type below to start chatting",
@@ -65,6 +67,7 @@ export const enUS: TranslationMap = {
   // Session messages
   "session.changeDetected": "👁️ Scene change detected",
   "session.recognizing": "(Recognizing speech...)",
+  "session.sttFailed": "(Speech recognition failed, please try again)",
   "session.noSpeech": "(No speech detected)",
   "session.interrupted": "(Interrupted)",