refactor(admin): 管理后台 UI - 独立设计系统并与用户端隔离

把后台 token、Ant 主题、壳层控件和抽屉弹窗从用户工作台拆开，去掉树状侧栏装饰线，色板仍用原来的中性黑白台阶。

Co-authored-by: Cursor <cursoragent@cursor.com>

web/src/pages/admin/channels/channels-page.tsx

@@ -6,7 +6,7 @@ import { useEffect, useRef, useState } from "react";
 import { useSearchParams } from "react-router";
 
 import { ChannelHeadersEditor, validateChannelHeaders } from "@/components/channel-headers-editor";
-import { PaginationBar } from "@/components/layout/workspace-page";
+import { PaginationBar } from "@/pages/admin/components/admin-ui";
 import { useDebouncedValue } from "@/hooks/use-debounced-value";
 import { refreshSystemChannels } from "@/lib/user-session";
 import { createAdminChannel, deleteAdminChannel, duplicateAdminChannel, listAdminChannels, updateAdminChannel } from "@/services/api/auth";