feat: 前端 UI 国际化，支持中/英/日三语切换

之前语言设置只影响后端 AI 行为，前端 UI 文字始终为中文。
新增轻量 i18n 模块（React Context + 翻译字典），切换语言后所有 UI 文字即时刷新。

- 新增 i18n 核心模块和 zh-CN/en-US/ja-JP 翻译文件（约 72 个 key）
- App.tsx 拆分为 App（Provider）+ AppContent（业务），确保 Hook 可访问 Context
- 所有组件通过 useI18n().t() 获取翻译文本
- errors.ts 改为接受翻译函数参数
- storage.ts saveConfig 时派发事件通知 locale 变化

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

@@ -0,0 +1,86 @@
+import type { TranslationMap } from "./index";
+
+export const zhCN: TranslationMap = {
+  // App header
+  "app.title": "AI 视觉对话助手",
+  "app.stats.requests": "次请求",
+
+  // Connection status
+  "status.connected": "已连接",
+  "status.connecting": "连接中...",
+  "status.disconnected": "未连接",
+
+  // Settings
+  "settings.title": "设置",
+  "settings.appearance": "外观",
+  "settings.theme": "主题",
+  "settings.theme.desc": "切换明暗主题",
+  "settings.theme.dark": "深色",
+  "settings.theme.light": "浅色",
+  "settings.session": "会话",
+  "settings.tts": "语音回答",
+  "settings.tts.desc": "AI 回答时同步播放语音",
+  "settings.detail": "图像精度",
+  "settings.detail.desc": "高精度适合识别文字，低精度省流量",
+  "settings.detail.low": "低",
+  "settings.detail.high": "高",
+  "settings.language": "语言",
+  "settings.language.desc": "交互语言偏好",
+
+  // 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...",
+  "video.clickToStart": "点击下方按钮开始对话",
+  "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.cameraOff": "关闭摄像头",
+  "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",
+
+  // 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.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.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",
+  "chat.input.placeholder": "输入文字对话...",
+  "chat.send": "发送",
+
+  // Session messages
+  "session.changeDetected": "👁️ 画面变化检测",
+  "session.recognizing": "（语音识别中...）",
+  "session.noSpeech": "（未识别到语音）",
+  "session.interrupted": "（已打断）",
+
+  // Errors
+  "error.INVALID_MESSAGE": "消息格式异常，请重试",
+  "error.SESSION_NOT_FOUND": "会话已过期，请重新连接",
+  "error.RATE_LIMITED": "请求太频繁，请稍后再试",
+  "error.IMAGE_TOO_LARGE": "图像过大，请降低分辨率",
+  "error.AUDIO_TOO_SHORT": "语音太短，请再说一句",
+  "error.LLM_TIMEOUT": "AI 响应超时，请重试",
+  "error.LLM_ERROR": "AI 服务异常，请稍后重试",
+  "error.STT_ERROR": "语音识别失败，请重试",
+  "error.TTS_ERROR": "语音合成失败",
+  "error.INTERNAL_ERROR": "服务内部错误，请重试",
+  "error.unknown": "未知错误",
+
+  // Device errors
+  "error.vadInit": "VAD 初始化失败",
+  "error.cameraAccess": "无法访问摄像头",
+  "error.micAccess": "无法访问麦克风",
+};