侧边栏增加 admin 用户管理入口

admin 用户可在侧边栏底部进入用户管理页，
支持新增/删除用户。

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

serve/frontend/src/components/Layout.tsx

@@ -28,6 +28,8 @@ import {
   Moon,
   Sun,
   User,
+  Users,
+  Shield,
   LogOut,
 } from 'lucide-react'
 import { Logo } from './Logo'
@@ -92,6 +94,7 @@ export function Layout() {
   })
 
   const username = authData?.username
+  const role = authData?.role
   const navigate = useNavigate()
   const qc = useQueryClient()
   const [darkMode, setDarkMode] = useState(() => {
@@ -209,6 +212,21 @@ export function Layout() {
             </div>
           )}
 
+          {/* 用户管理 (admin only) */}
+          {role === 'admin' && (
+            <NavLink
+              to="/settings/users"
+              className={({ isActive }) =>
+                'flex items-center gap-2 px-3 py-1.5 text-xs text-foreground/70 hover:bg-elevated hover:text-foreground transition-colors' +
+                (isActive ? ' bg-accent/10 text-accent' : '')
+              }
+            >
+              <Users className="h-3.5 w-3.5" />
+              <Shield className="h-3 w-3 text-amber-400" />
+              <span className="flex-1">用户管理</span>
+            </NavLink>
+          )}
+
           {/* 暗夜模式 */}
           <div className="px-1">
             <button