移除活跃Tab下划线渐变动画

- 删除 .nav-tab-active CSS 及 ::after 伪元素
- 移除 NavLink 中的 nav-tab-active 类名
- 活跃Tab仅保留左侧彩色竖条 + 背景高亮

Co-Authored-By: new-provider/LongCat-2.0 <<EMAIL>>
)

frontend/src/admin/AdminLayout.tsx

@@ -227,9 +227,9 @@ export default function AdminLayout() {
             to="/admin"
             end
             className={({ isActive }) =>
-              `flex min-h-[40px] items-center gap-2.5 border-l-4 px-3 pb-1 text-sm transition-all duration-300 ${
+              `flex min-h-[40px] items-center gap-2.5 border-l-4 px-3 text-sm transition-all duration-300 ${
                 isActive
-                  ? 'border-press bg-press-wash/60 font-medium text-press nav-tab-active'
+                  ? 'border-press bg-press-wash/60 font-medium text-press'
                   : 'border-transparent text-ink-500 hover:bg-paper-100 hover:text-ink-900'
               }`
             }