feat: 统一 Admin 用户中心 + 修复默认对话记忆存储路径

## 前端
- 新增 AdminUserHub 页面，合并用户管理/审计/记忆三个页面为统一中心（左侧用户列表 + 右侧 Tab）
- 旧三条 admin 路由重定向到新 /admin 路由
- 侧边栏导航菜单合并为单一入口"用户中心"
- memory-settings-page: 默认助手下拉选项传 agent=default（后端识别为全局哨兵）

## 后端
- memory.py: 将 "default" 加入全局哨兵 _GLOBAL_AGENT_VALUES，?agent=default 读全局记忆文件
- memory_middleware.py: _resolve_agent_id() 无 agent 上下文时返回 None，默认对话写入 users/{id}/memory.json
- manager.py: 移除 agent_id or "default" 回退，让 None 透传给 V1 provider
- admin_users.py: Admin 记忆 API 将"默认助手"桶改为读全局文件（agent_id=None）

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend-web/src/pages/AdminUserHubPage.tsx

@@ -0,0 +1,5 @@
+import { AdminUserHub } from "@/components/workspace/admin/admin-user-hub";
+
+export default function AdminUserHubPage() {
+  return <AdminUserHub />;
+}

frontend-web/src/pages/WorkspaceRoutes.tsx

@@ -29,6 +29,7 @@ import SkillsPage from "./SkillsPage";
 import UsersPage from "./UsersPage";
 import AdminUserActivityPage from "./AdminUserActivityPage";
 import AdminMemoryPage from "./AdminMemoryPage";
+import AdminUserHubPage from "./AdminUserHubPage";
 import TagsPage from "./TagsPage";
 import RecommendedQuestionsPage from "./RecommendedQuestionsPage";
 import AIWritingPage from "@/open-canvas/pages/AIWritingPage";
@@ -206,18 +207,32 @@ export default function WorkspaceRoutes({
           </WorkspaceLayout>
         }
       />
+      {/* Unified admin user hub */}
       <Route
-        path="admin/users"
+        path="admin"
         element={
           getStoredAuth()?.system_role === "admin" ? (
-            <WorkspaceLayout {...layoutProps}>
-              <UsersPage />
+            <WorkspaceLayout {...layoutProps} pageSidebarHidden>
+              <AdminUserHubPage />
             </WorkspaceLayout>
           ) : (
             <Navigate to="/page/workspace/chats/new" replace />
           )
         }
       />
+      {/* Legacy redirects */}
+      <Route
+        path="admin/users"
+        element={<Navigate to="/page/workspace/admin" replace />}
+      />
+      <Route
+        path="admin/user-activity"
+        element={<Navigate to="/page/workspace/admin" replace />}
+      />
+      <Route
+        path="admin/memory"
+        element={<Navigate to="/page/workspace/admin" replace />}
+      />
       <Route
         path="admin/scheduled-tasks"
         element={
@@ -230,30 +245,6 @@ export default function WorkspaceRoutes({
           )
         }
       />
-      <Route
-        path="admin/user-activity"
-        element={
-          getStoredAuth()?.system_role === "admin" ? (
-            <WorkspaceLayout {...layoutProps}>
-              <AdminUserActivityPage />
-            </WorkspaceLayout>
-          ) : (
-            <Navigate to="/page/workspace/chats/new" replace />
-          )
-        }
-      />
-      <Route
-        path="admin/memory"
-        element={
-          getStoredAuth()?.system_role === "admin" ? (
-            <WorkspaceLayout {...layoutProps}>
-              <AdminMemoryPage />
-            </WorkspaceLayout>
-          ) : (
-            <Navigate to="/page/workspace/chats/new" replace />
-          )
-        }
-      />
       <Route
         path="admin/tags"
         element={