feat: 新增会话历史侧边栏，支持新建/切换/删除/重命名会话

类似 ChatGPT 的侧边栏布局，消息历史持久化到 localStorage。

- 新增 SessionSummary 类型和 localStorage 读写函数
- 新增 useSessionList Hook 管理会话列表 CRUD
- 新增 SessionSidebar 组件（展开/折叠、行内重命名）
- App.tsx 协调 useSessionList 和 useVisionSession
- 消息变化时自动持久化，切换/新建会话时保存并加载
- 新增 i18n 翻译 key（中/英/日）

frontend/src/hooks/useSessionList.ts

@@ -0,0 +1,117 @@
+// ============================================================
+// useSessionList — 会话历史列表管理
+// 职责：会话 CRUD、消息持久化、切换会话
+// ============================================================
+
+import { useCallback, useEffect, useRef, useState } from "react";
+import { v4 as uuidv4 } from "uuid";
+import {
+  loadSessionSummaries,
+  saveSessionSummaries,
+  loadSessionMessages,
+  saveSessionMessages,
+  deleteSessionMessages,
+} from "../lib/storage";
+import type { ChatMessage, SessionSummary } from "../types";
+
+/** 截取预览文本 */
+function getPreview(text: string, maxLen = 50): string {
+  const clean = text.replace(/[\n\r]/g, " ").trim();
+  return clean.length > maxLen ? clean.slice(0, maxLen) + "…" : clean;
+}
+
+export function useSessionList() {
+  const [sessions, setSessions] = useState<SessionSummary[]>(() => loadSessionSummaries());
+  const [activeSessionId, setActiveSessionId] = useState<string | null>(null);
+  const sessionsRef = useRef(sessions);
+  useEffect(() => { sessionsRef.current = sessions; }, [sessions]);
+
+  /** 创建新会话 */
+  const createSession = useCallback((): string => {
+    const id = uuidv4();
+    const now = Date.now();
+    const summary: SessionSummary = {
+      id,
+      title: "新对话",
+      createdAt: now,
+      lastActiveAt: now,
+      messageCount: 0,
+      preview: "",
+    };
+    setSessions((prev) => [summary, ...prev]);
+    setActiveSessionId(id);
+    // 持久化
+    const all = [summary, ...sessionsRef.current];
+    saveSessionSummaries(all);
+    return id;
+  }, []);
+
+  /** 删除会话 */
+  const deleteSession = useCallback((id: string) => {
+    setSessions((prev) => prev.filter((s) => s.id !== id));
+    deleteSessionMessages(id);
+    const remaining = sessionsRef.current.filter((s) => s.id !== id);
+    saveSessionSummaries(remaining);
+    // 如果删除的是当前会话，清空 active
+    setActiveSessionId((prev) => (prev === id ? null : prev));
+  }, []);
+
+  /** 重命名会话 */
+  const renameSession = useCallback((id: string, title: string) => {
+    setSessions((prev) => prev.map((s) => (s.id === id ? { ...s, title } : s)));
+    const updated = sessionsRef.current.map((s) => (s.id === id ? { ...s, title } : s));
+    saveSessionSummaries(updated);
+  }, []);
+
+  /** 保存当前会话的消息历史 */
+  const saveCurrentSession = useCallback((messages: ChatMessage[]) => {
+    const id = sessionsRef.current.length > 0 ? sessionsRef.current[0].id : null;
+    // 找到 activeSessionId 对应的会话
+    // 这里不依赖 activeSessionId state，而是通过参数传入
+    return messages;
+  }, []);
+
+  /** 保存指定会话的消息并更新摘要 */
+  const persistSession = useCallback((sessionId: string, messages: ChatMessage[]) => {
+    if (!sessionId) return;
+    saveSessionMessages(sessionId, messages);
+    // 更新摘要
+    const firstUserMsg = messages.find((m) => m.role === "user");
+    const title = firstUserMsg ? getPreview(firstUserMsg.content, 20) : "新对话";
+    const lastMsg = messages[messages.length - 1];
+    const summary: Partial<SessionSummary> = {
+      title,
+      messageCount: messages.length,
+      lastActiveAt: lastMsg?.timestamp || Date.now(),
+      preview: lastMsg ? getPreview(lastMsg.content) : "",
+    };
+    setSessions((prev) => {
+      const updated = prev.map((s) => (s.id === sessionId ? { ...s, ...summary } : s));
+      saveSessionSummaries(updated);
+      return updated;
+    });
+  }, []);
+
+  /** 加载指定会话的消息历史 */
+  const loadMessages = useCallback((sessionId: string): ChatMessage[] => {
+    return loadSessionMessages(sessionId);
+  }, []);
+
+  /** 选择会话（返回需要加载的消息） */
+  const selectSession = useCallback((id: string): ChatMessage[] => {
+    setActiveSessionId(id);
+    return loadSessionMessages(id);
+  }, []);
+
+  return {
+    sessions,
+    activeSessionId,
+    setActiveSessionId,
+    createSession,
+    deleteSession,
+    renameSession,
+    persistSession,
+    loadMessages,
+    selectSession,
+  };
+}

frontend/src/hooks/useVisionSession.ts

@@ -458,6 +458,7 @@ export function useVisionSession() {
 
   return {
     messages,
+    setMessages,
     currentReply,
     isProcessing,
     isAudioPlaying,