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/ChatPanel/index.tsx

@@ -4,6 +4,7 @@
 // ============================================================
 
 import { useEffect, useRef, useState } from "react";
+import { useI18n } from "../../lib/i18n";
 import type { ChatMessage } from "../../types";
 import type { ConnectionStatus } from "../../lib/websocket";
 
@@ -19,6 +20,7 @@ export function ChatPanel({ messages, currentReply, connectionStatus, onSendText
   const containerRef = useRef<HTMLDivElement>(null);
   const isAutoScroll = useRef(true);
   const [inputText, setInputText] = useState("");
+  const { t } = useI18n();
 
   const isConnected = connectionStatus === "connected";
 
@@ -56,8 +58,8 @@ export function ChatPanel({ messages, currentReply, connectionStatus, onSendText
     return (
       <div className="chat-panel chat-panel--empty">
         <span className="chat-panel--empty-icon">💬</span>
-        <p>点击下方按钮开始对话</p>
-        <span className="chat-panel--empty-hint">连接后可打字或语音与 AI 交互</span>
+        <p>{t("chat.empty.prompt")}</p>
+        <span className="chat-panel--empty-hint">{t("chat.empty.hint")}</span>
       </div>
     );
   }
@@ -69,15 +71,15 @@ export function ChatPanel({ messages, currentReply, connectionStatus, onSendText
         {messages.length === 0 && !currentReply && (
           <div className="chat-panel__welcome">
             <span className="chat-panel__welcome-icon">💬</span>
-            <p>在下方输入文字开始对话</p>
-            <span className="chat-panel__welcome-hint">也可以开启麦克风用语音对话</span>
+            <p>{t("chat.welcome.prompt")}</p>
+            <span className="chat-panel__welcome-hint">{t("chat.welcome.hint")}</span>
           </div>
         )}
 
         {messages.map((msg, index) => (
           <div key={index} className={`chat-message chat-message--${msg.role}`}>
             <div className="chat-message__role">
-              {msg.role === "user" ? "你" : "AI"}
+              {msg.role === "user" ? t("chat.userLabel") : "AI"}
             </div>
             <div className="chat-message__content">{msg.content}</div>
             {msg.role === "assistant" && msg.tokensUsed !== undefined && (
@@ -110,7 +112,7 @@ export function ChatPanel({ messages, currentReply, connectionStatus, onSendText
           <input
             type="text"
             className="chat-input__field"
-            placeholder="输入文字对话..."
+            placeholder={t("chat.input.placeholder")}
             value={inputText}
             onChange={(e) => setInputText(e.target.value)}
           />
@@ -118,7 +120,7 @@ export function ChatPanel({ messages, currentReply, connectionStatus, onSendText
             type="submit"
             className="chat-input__send"
             disabled={!inputText.trim()}
-            title="发送"
+            title={t("chat.send")}
           >
             ➤
           </button>