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/components/ConfigPanel/index.tsx

@@ -3,6 +3,7 @@
 // 职责：主题切换、TTS 开关、detail level 切换、语言选择
 // ============================================================
 
+import { useI18n } from "../../lib/i18n";
 import type { SessionConfig, Theme } from "../../types";
 
 interface ConfigPanelProps {
@@ -14,40 +15,42 @@ interface ConfigPanelProps {
 }
 
 export function ConfigPanel({ config, theme, onUpdate, onThemeChange, onClose }: ConfigPanelProps) {
+  const { t } = useI18n();
+
   return (
     <div className="drawer-overlay" onClick={onClose}>
       <div className="drawer" onClick={(e) => e.stopPropagation()}>
         <div className="drawer__header">
-          <span className="drawer__title">设置</span>
+          <span className="drawer__title">{t("settings.title")}</span>
           <button className="drawer__close" onClick={onClose}>✕</button>
         </div>
 
         <div className="drawer__body">
           <div className="config-group">
-            <div className="config-group__title">外观</div>
+            <div className="config-group__title">{t("settings.appearance")}</div>
 
             <label className="config-row">
               <div className="config-row__info">
-                <span className="config-row__label">主题</span>
-                <span className="config-row__desc">切换明暗主题</span>
+                <span className="config-row__label">{t("settings.theme")}</span>
+                <span className="config-row__desc">{t("settings.theme.desc")}</span>
               </div>
               <select
                 value={theme}
                 onChange={(e) => onThemeChange(e.target.value as Theme)}
               >
-                <option value="dark">深色</option>
-                <option value="light">浅色</option>
+                <option value="dark">{t("settings.theme.dark")}</option>
+                <option value="light">{t("settings.theme.light")}</option>
               </select>
             </label>
           </div>
 
           <div className="config-group">
-            <div className="config-group__title">会话</div>
+            <div className="config-group__title">{t("settings.session")}</div>
 
             <label className="config-row">
               <div className="config-row__info">
-                <span className="config-row__label">语音回答</span>
-                <span className="config-row__desc">AI 回答时同步播放语音</span>
+                <span className="config-row__label">{t("settings.tts")}</span>
+                <span className="config-row__desc">{t("settings.tts.desc")}</span>
               </div>
               <input
                 type="checkbox"
@@ -58,22 +61,22 @@ export function ConfigPanel({ config, theme, onUpdate, onThemeChange, onClose }:
 
             <label className="config-row">
               <div className="config-row__info">
-                <span className="config-row__label">图像精度</span>
-                <span className="config-row__desc">高精度适合识别文字，低精度省流量</span>
+                <span className="config-row__label">{t("settings.detail")}</span>
+                <span className="config-row__desc">{t("settings.detail.desc")}</span>
               </div>
               <select
                 value={config.detailLevel}
                 onChange={(e) => onUpdate({ detailLevel: e.target.value as "low" | "high" })}
               >
-                <option value="low">低</option>
-                <option value="high">高</option>
+                <option value="low">{t("settings.detail.low")}</option>
+                <option value="high">{t("settings.detail.high")}</option>
               </select>
             </label>
 
             <label className="config-row">
               <div className="config-row__info">
-                <span className="config-row__label">语言</span>
-                <span className="config-row__desc">交互语言偏好</span>
+                <span className="config-row__label">{t("settings.language")}</span>
+                <span className="config-row__desc">{t("settings.language.desc")}</span>
               </div>
               <select
                 value={config.language}