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",

frontend/src/lib/i18n/ja-JP.ts

@@ -33,12 +33,14 @@ export const jaJP: TranslationMap = {
   "video.playing": "🔊 再生中...",
   "video.initVad": "音声検出を初期化中...",
   "video.clickToStart": "下のボタンをクリックして開始",
+  "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",
 
   // Controls
   "controls.connecting": "接続中...",
   "controls.start": "🎙️ 対話を開始",
+  "controls.startVideo": "🎙️ ビデオ通話を開始",
   "controls.cameraOff": "カメラをオフ",
   "controls.cameraOn": "カメラをオン",
   "controls.micOff": "マイクをオフ",
@@ -52,8 +54,8 @@ export const jaJP: TranslationMap = {
   "chat.title": "チャット",
   "chat.mode.observation": "観察モード",
   "chat.reconnecting": "接続が切断されました。再接続中...",
-  "chat.empty.prompt": "下のボタンをクリックして開始",
-  "chat.empty.hint": "接続後にテキストまたは音声でAIと対話できます",
+  "chat.empty.prompt": "下にテキストを入力して対話を開始",
+  "chat.empty.hint": "テキストでAIと対話、または左のボタンでビデオ通話を開始",
   "chat.welcome.prompt": "下にテキストを入力して対話を開始",
   "chat.welcome.hint": "マイクを有効にして音声対話もできます",
   "chat.userLabel": "あなた",

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

@@ -33,12 +33,14 @@ export const zhCN: TranslationMap = {
   "video.playing": "🔊 正在播放...",
   "video.initVad": "正在初始化语音检测...",
   "video.clickToStart": "点击下方按钮开始对话",
+  "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",
 
   // Controls
   "controls.connecting": "连接中...",
   "controls.start": "🎙️ 开始对话",
+  "controls.startVideo": "🎙️ 开始视频通话",
   "controls.cameraOff": "关闭摄像头",
   "controls.cameraOn": "开启摄像头",
   "controls.micOff": "关闭麦克风",
@@ -52,8 +54,8 @@ export const zhCN: TranslationMap = {
   "chat.title": "对话",
   "chat.mode.observation": "观察模式",
   "chat.reconnecting": "连接已断开，正在重连...",
-  "chat.empty.prompt": "点击下方按钮开始对话",
-  "chat.empty.hint": "连接后可打字或语音与 AI 交互",
+  "chat.empty.prompt": "在下方输入文字开始对话",
+  "chat.empty.hint": "输入文字即可与 AI 交互，也可点击左侧按钮开启视频",
   "chat.welcome.prompt": "在下方输入文字开始对话",
   "chat.welcome.hint": "也可以开启麦克风用语音对话",
   "chat.userLabel": "你",