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/index.tsx

@@ -1,6 +1,8 @@
 import { useUserStore } from "@/stores/use-user-store";
 import { AdminProvider } from "./admin-context";
 import { AdminShell } from "./components/admin-shell";
+import "./theme/admin-tokens.css";
+import "./theme/admin-chrome.css";
 
 export default function AdminPage() {
     const actor = useUserStore((state) => state.user);
@@ -9,10 +11,10 @@ export default function AdminPage() {
     if (!hydrated) return null;
     if (actor?.role !== "admin") {
         return (
-            <main className="app-workspace-page min-h-dvh px-6 py-10 text-foreground">
-                <div className="mx-auto max-w-3xl rounded-lg border border-border bg-[var(--workspace-surface)] p-6">
-                    <h1 className="text-2xl font-semibold">无权限</h1>
-                    <p className="mt-2 text-sm text-foreground/55">当前账号不是管理员，无法访问后台。</p>
+            <main data-admin-root className="admin-unauthorized">
+                <div className="admin-unauthorized-card">
+                    <h1>无权限</h1>
+                    <p>当前账号不是管理员，无法访问后台。</p>
                 </div>
             </main>
         );