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/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;