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/lib/i18n/en-US.ts

@@ -81,6 +81,15 @@ export const enUS: TranslationMap = {
   "error.INTERNAL_ERROR": "Internal server error, please retry",
   "error.unknown": "Unknown error",
 
+  // Sidebar
+  "sidebar.new": "New Chat",
+  "sidebar.expand": "Expand sidebar",
+  "sidebar.collapse": "Collapse sidebar",
+  "sidebar.empty": "No chat history",
+  "sidebar.messages": " messages",
+  "sidebar.rename": "Rename",
+  "sidebar.delete": "Delete",
+
   // Device errors
   "error.vadInit": "VAD initialization failed",
   "error.cameraAccess": "Cannot access camera",

frontend/src/lib/i18n/ja-JP.ts

@@ -81,6 +81,15 @@ export const jaJP: TranslationMap = {
   "error.INTERNAL_ERROR": "サーバー内部エラー。再試行してください",
   "error.unknown": "不明なエラー",
 
+  // Sidebar
+  "sidebar.new": "新しいチャット",
+  "sidebar.expand": "サイドバーを展開",
+  "sidebar.collapse": "サイドバーを折りたたむ",
+  "sidebar.empty": "会話履歴がありません",
+  "sidebar.messages": "件のメッセージ",
+  "sidebar.rename": "名前を変更",
+  "sidebar.delete": "削除",
+
   // Device errors
   "error.vadInit": "VAD初期化に失敗しました",
   "error.cameraAccess": "カメラにアクセスできません",

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

@@ -81,6 +81,15 @@ export const zhCN: TranslationMap = {
   "error.INTERNAL_ERROR": "服务内部错误，请重试",
   "error.unknown": "未知错误",
 
+  // Sidebar
+  "sidebar.new": "新对话",
+  "sidebar.expand": "展开侧边栏",
+  "sidebar.collapse": "收起侧边栏",
+  "sidebar.empty": "暂无会话记录",
+  "sidebar.messages": "条消息",
+  "sidebar.rename": "重命名",
+  "sidebar.delete": "删除",
+
   // Device errors
   "error.vadInit": "VAD 初始化失败",
   "error.cameraAccess": "无法访问摄像头",

frontend/src/lib/storage.ts

@@ -3,7 +3,7 @@
 // 职责：会话配置持久化
 // ============================================================
 
-import type { SessionConfig, Theme } from "../types";
+import type { ChatMessage, SessionConfig, SessionSummary, Theme } from "../types";
 
 const CONFIG_KEY = "camtalk:config";
 const THEME_KEY = "camtalk:theme";
@@ -52,3 +52,51 @@ export function saveTheme(theme: Theme): void {
     localStorage.setItem(THEME_KEY, theme);
   } catch { /* ignore */ }
 }
+
+// ---- 会话历史存储 ----
+
+const SESSIONS_KEY = "camtalk:sessions";
+const SESSION_MSG_PREFIX = "camtalk:session:";
+
+/** 加载所有会话摘要（按 lastActiveAt 倒序） */
+export function loadSessionSummaries(): SessionSummary[] {
+  try {
+    const raw = localStorage.getItem(SESSIONS_KEY);
+    if (!raw) return [];
+    return (JSON.parse(raw) as SessionSummary[]).sort((a, b) => b.lastActiveAt - a.lastActiveAt);
+  } catch {
+    return [];
+  }
+}
+
+/** 保存会话摘要列表 */
+export function saveSessionSummaries(sessions: SessionSummary[]): void {
+  try {
+    localStorage.setItem(SESSIONS_KEY, JSON.stringify(sessions));
+  } catch { /* ignore */ }
+}
+
+/** 加载单个会话的消息历史 */
+export function loadSessionMessages(sessionId: string): ChatMessage[] {
+  try {
+    const raw = localStorage.getItem(SESSION_MSG_PREFIX + sessionId);
+    if (!raw) return [];
+    return JSON.parse(raw) as ChatMessage[];
+  } catch {
+    return [];
+  }
+}
+
+/** 保存单个会话的消息历史 */
+export function saveSessionMessages(sessionId: string, messages: ChatMessage[]): void {
+  try {
+    localStorage.setItem(SESSION_MSG_PREFIX + sessionId, JSON.stringify(messages));
+  } catch { /* ignore */ }
+}
+
+/** 删除单个会话的消息历史 */
+export function deleteSessionMessages(sessionId: string): void {
+  try {
+    localStorage.removeItem(SESSION_MSG_PREFIX + sessionId);
+  } catch { /* ignore */ }
+}