feat: 聊天框与视频功能解耦，输入文字即可开始对话

之前必须点击「开始对话」连接 WebSocket 后才能使用聊天框。
现在聊天输入框始终可用，输入文字自动连接 WebSocket 并发送消息；
左侧按钮改为「开始视频通话」，单独控制摄像头/麦克风。

- ChatPanel 移除 !isConnected early return，始终显示输入框
- useVisionSession 新增待发消息队列，sendTextMessage 支持自动连接
- startSession 改为仅负责视频（摄像头 + 麦克风 + VAD）
- 修复 statusRef 在 useWebSocketManager 之前声明导致的 TDZ 错误
- 更新 i18n 翻译（新增 controls.startVideo、video.placeholder）

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

@@ -33,12 +33,14 @@ export const enUS: TranslationMap = {
   "video.playing": "🔊 Playing...",
   "video.initVad": "Initializing voice detection...",
   "video.clickToStart": "Click the button below to start",
+  "video.placeholder": "Type in the chat panel to start",
   "video.cameraOff": "Camera is off",
   "video.cameraOff.hint": "You can type in the chat panel",
 
   // Controls
   "controls.connecting": "Connecting...",
   "controls.start": "🎙️ Start Session",
+  "controls.startVideo": "🎙️ Start Video Call",
   "controls.cameraOff": "Turn off camera",
   "controls.cameraOn": "Turn on camera",
   "controls.micOff": "Turn off microphone",
@@ -52,8 +54,8 @@ export const enUS: TranslationMap = {
   "chat.title": "Chat",
   "chat.mode.observation": "Observing",
   "chat.reconnecting": "Connection lost, reconnecting...",
-  "chat.empty.prompt": "Click the button below to start",
-  "chat.empty.hint": "Type or speak with AI after connecting",
+  "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",
   "chat.welcome.hint": "Or enable microphone for voice chat",
   "chat.userLabel": "You",