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/components/SessionSidebar/index.tsx

@@ -0,0 +1,160 @@
+// ============================================================
+// SessionSidebar — 会话历史侧边栏
+// 职责：展示会话列表、新建/切换/删除/重命名会话
+// ============================================================
+
+import { useState } from "react";
+import { useI18n } from "../../lib/i18n";
+import type { SessionSummary } from "../../types";
+
+interface SessionSidebarProps {
+  sessions: SessionSummary[];
+  activeSessionId: string | null;
+  collapsed: boolean;
+  onToggleCollapse: () => void;
+  onNewSession: () => void;
+  onSelectSession: (id: string) => void;
+  onDeleteSession: (id: string) => void;
+  onRenameSession: (id: string, title: string) => void;
+}
+
+/** 格式化相对时间 */
+function formatRelativeTime(ts: number): string {
+  const now = Date.now();
+  const diff = now - ts;
+  const min = 60 * 1000;
+  const hour = 60 * min;
+  const day = 24 * hour;
+
+  if (diff < min) return "刚刚";
+  if (diff < hour) return `${Math.floor(diff / min)}分钟前`;
+  if (diff < day) return `${Math.floor(diff / hour)}小时前`;
+  if (diff < 2 * day) return "昨天";
+  if (diff < 7 * day) return `${Math.floor(diff / day)}天前`;
+  const d = new Date(ts);
+  return `${d.getMonth() + 1}/${d.getDate()}`;
+}
+
+export function SessionSidebar({
+  sessions,
+  activeSessionId,
+  collapsed,
+  onToggleCollapse,
+  onNewSession,
+  onSelectSession,
+  onDeleteSession,
+  onRenameSession,
+}: SessionSidebarProps) {
+  const { t } = useI18n();
+  const [editingId, setEditingId] = useState<string | null>(null);
+  const [editTitle, setEditTitle] = useState("");
+
+  const handleStartRename = (id: string, currentTitle: string) => {
+    setEditingId(id);
+    setEditTitle(currentTitle);
+  };
+
+  const handleConfirmRename = () => {
+    if (editingId && editTitle.trim()) {
+      onRenameSession(editingId, editTitle.trim());
+    }
+    setEditingId(null);
+    setEditTitle("");
+  };
+
+  const handleCancelRename = () => {
+    setEditingId(null);
+    setEditTitle("");
+  };
+
+  if (collapsed) {
+    return (
+      <div className="sidebar sidebar--collapsed">
+        <button className="sidebar__toggle" onClick={onToggleCollapse} title={t("sidebar.expand")}>
+          ☰
+        </button>
+        <button className="sidebar__new-btn" onClick={onNewSession} title={t("sidebar.new")}>
+          ✚
+        </button>
+      </div>
+    );
+  }
+
+  return (
+    <div className="sidebar">
+      <div className="sidebar__header">
+        <button className="sidebar__new-btn sidebar__new-btn--full" onClick={onNewSession}>
+          ✚ {t("sidebar.new")}
+        </button>
+        <button className="sidebar__toggle" onClick={onToggleCollapse} title={t("sidebar.collapse")}>
+          ◀
+        </button>
+      </div>
+
+      <div className="sidebar__list">
+        {sessions.length === 0 ? (
+          <div className="sidebar__empty">{t("sidebar.empty")}</div>
+        ) : (
+          sessions.map((session) => (
+            <div
+              key={session.id}
+              className={`sidebar__item ${session.id === activeSessionId ? "sidebar__item--active" : ""}`}
+              onClick={() => onSelectSession(session.id)}
+            >
+              {editingId === session.id ? (
+                <div className="sidebar__edit" onClick={(e) => e.stopPropagation()}>
+                  <input
+                    className="sidebar__edit-input"
+                    value={editTitle}
+                    onChange={(e) => setEditTitle(e.target.value)}
+                    onKeyDown={(e) => {
+                      if (e.key === "Enter") handleConfirmRename();
+                      if (e.key === "Escape") handleCancelRename();
+                    }}
+                    autoFocus
+                  />
+                  <button className="sidebar__edit-btn" onClick={handleConfirmRename}>✓</button>
+                  <button className="sidebar__edit-btn" onClick={handleCancelRename}>✕</button>
+                </div>
+              ) : (
+                <>
+                  <div className="sidebar__item-content">
+                    <div className="sidebar__item-title">{session.title}</div>
+                    <div className="sidebar__item-meta">
+                      {session.messageCount > 0 && (
+                        <span>{session.messageCount}{t("sidebar.messages")}</span>
+                      )}
+                      <span>{formatRelativeTime(session.lastActiveAt)}</span>
+                    </div>
+                  </div>
+                  <div className="sidebar__item-actions">
+                    <button
+                      className="sidebar__action-btn"
+                      onClick={(e) => {
+                        e.stopPropagation();
+                        handleStartRename(session.id, session.title);
+                      }}
+                      title={t("sidebar.rename")}
+                    >
+                      ✏
+                    </button>
+                    <button
+                      className="sidebar__action-btn sidebar__action-btn--danger"
+                      onClick={(e) => {
+                        e.stopPropagation();
+                        onDeleteSession(session.id);
+                      }}
+                      title={t("sidebar.delete")}
+                    >
+                      🗑
+                    </button>
+                  </div>
+                </>
+              )}
+            </div>
+          ))
+        )}
+      </div>
+    </div>
+  );
+}