fix: expose staging admin entry

Use persisted self-hosted roles and the isolated admin origin so authorized staging accounts can discover the protected admin surface.

frontend/src/components/app-sidebar.tsx

@@ -38,6 +38,7 @@ export type SidebarAccount = {
   email: string;
   credits: number;
   isAdmin: boolean;
+  adminUrl: string | null;
   initial: string;
 };
 
@@ -214,8 +215,8 @@ export function AppSidebar({
                 <button className="account-menu-item" role="menuitem" type="button" onClick={() => handleAccountAction(onOpenRedeem)}>
                   <Gift aria-hidden="true" /><span>兑换点数</span><small>{account.credits} 点</small>
                 </button>
-                {account.isAdmin && <Link className="account-menu-item" href="/admin/codes" role="menuitem" onClick={() => onAccountMenuOpenChange(false)}>
-                  <KeyRound aria-hidden="true" /><span>管理兑换码</span><ChevronRight aria-hidden="true" />
+                {account.isAdmin && account.adminUrl && <Link className="account-menu-item" href={account.adminUrl} role="menuitem" onClick={() => onAccountMenuOpenChange(false)}>
+                  <KeyRound aria-hidden="true" /><span>后台管理</span><ChevronRight aria-hidden="true" />
                 </Link>}
                 <div className="account-menu-separator" role="separator" />
                 <button className="account-menu-item account-menu-danger" role="menuitem" type="button" onClick={() => handleAccountAction(onOpenLogout)}>