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/en-US.ts

@@ -0,0 +1,86 @@
+import type { TranslationMap } from "./index";
+
+export const enUS: TranslationMap = {
+  // App header
+  "app.title": "AI Vision Assistant",
+  "app.stats.requests": "requests",
+
+  // Connection status
+  "status.connected": "Connected",
+  "status.connecting": "Connecting...",
+  "status.disconnected": "Disconnected",
+
+  // Settings
+  "settings.title": "Settings",
+  "settings.appearance": "Appearance",
+  "settings.theme": "Theme",
+  "settings.theme.desc": "Switch between light and dark theme",
+  "settings.theme.dark": "Dark",
+  "settings.theme.light": "Light",
+  "settings.session": "Session",
+  "settings.tts": "Voice Response",
+  "settings.tts.desc": "Play voice along with AI response",
+  "settings.detail": "Image Quality",
+  "settings.detail.desc": "High quality for text recognition, low saves bandwidth",
+  "settings.detail.low": "Low",
+  "settings.detail.high": "High",
+  "settings.language": "Language",
+  "settings.language.desc": "Interaction language preference",
+
+  // Video indicators
+  "video.observing": "👁️ Observing",
+  "video.listening": "🎤 Listening...",
+  "video.playing": "🔊 Playing...",
+  "video.initVad": "Initializing voice detection...",
+  "video.clickToStart": "Click the button below 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.cameraOff": "Turn off camera",
+  "controls.cameraOn": "Turn on camera",
+  "controls.micOff": "Turn off microphone",
+  "controls.micOn": "Turn on microphone",
+  "controls.observing": "👁️ Observing",
+  "controls.observation": "👁️ Observe",
+  "controls.interrupt": "⏹ Interrupt",
+  "controls.stop": "End Session",
+
+  // Chat panel
+  "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.welcome.prompt": "Type below to start chatting",
+  "chat.welcome.hint": "Or enable microphone for voice chat",
+  "chat.userLabel": "You",
+  "chat.input.placeholder": "Type a message...",
+  "chat.send": "Send",
+
+  // Session messages
+  "session.changeDetected": "👁️ Scene change detected",
+  "session.recognizing": "(Recognizing speech...)",
+  "session.noSpeech": "(No speech detected)",
+  "session.interrupted": "(Interrupted)",
+
+  // Errors
+  "error.INVALID_MESSAGE": "Invalid message format, please retry",
+  "error.SESSION_NOT_FOUND": "Session expired, please reconnect",
+  "error.RATE_LIMITED": "Too many requests, please try again later",
+  "error.IMAGE_TOO_LARGE": "Image too large, please reduce resolution",
+  "error.AUDIO_TOO_SHORT": "Audio too short, please try again",
+  "error.LLM_TIMEOUT": "AI response timed out, please retry",
+  "error.LLM_ERROR": "AI service error, please try again later",
+  "error.STT_ERROR": "Speech recognition failed, please retry",
+  "error.TTS_ERROR": "Voice synthesis failed",
+  "error.INTERNAL_ERROR": "Internal server error, please retry",
+  "error.unknown": "Unknown error",
+
+  // Device errors
+  "error.vadInit": "VAD initialization failed",
+  "error.cameraAccess": "Cannot access camera",
+  "error.micAccess": "Cannot access microphone",
+};