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'
               }`
             }

frontend/src/index.css

@@ -16,25 +16,6 @@
   text-shadow: 0.5px 0.5px 0 currentColor;
 }
 
-/* 活跃Tab下划线渐变动画 */
-.nav-tab-active {
-  position: relative;
-}
-.nav-tab-active::after {
-  content: '';
-  position: absolute;
-  bottom: 0;
-  left: 50%;
-  width: 0;
-  height: 2px;
-  background: linear-gradient(90deg, transparent, #17140F, transparent);
-  transition: all 0.3s ease;
-  transform: translateX(-50%);
-}
-.nav-tab-active::after {
-  width: 80%;
-}
-
 /* 页面内容横向轻扫+淡入 */
 .page-content-enter {
   animation: pageSlideIn 0.3s ease-out;