fix(ui): 全站视觉审查修复 — 深色模式适配与交互优化

- 法律页面(条款/隐私) Hero 渐变背景深色模式适配，新增 --color-hero-dark-end 变量
- Badge secondary 变体深色模式下从白底白字改为主题自适应灰底
- 首页 Hero 快速导航图标 strokeWidth 加粗提升可读性 (1.8→2.2)
- 服务详情挑战卡片添加 border + hover 边框变色增强层次感
- 移动端 Tab Bar 激活指示器升级为顶部+底部双信号

src/components/layout/mobile-tab-bar.tsx

@@ -50,6 +50,13 @@ export function MobileTabBar() {
               className="flex flex-col items-center justify-center flex-1 h-full relative group min-h-12"
             >
               <div className="relative flex flex-col items-center justify-center py-2">
+                {active && (
+                  <motion.div
+                    layoutId="activeTabTop"
+                    className="absolute -top-0 w-6 h-[3px] bg-[var(--color-brand-primary)] rounded-full"
+                    transition={{ type: 'spring', stiffness: 380, damping: 30 }}
+                  />
+                )}
                 <Icon
                   className={cn(
                     'w-6 h-6 transition-colors',
@@ -66,7 +73,7 @@ export function MobileTabBar() {
                 </span>
                 {active && (
                   <motion.div
-                    layoutId="activeTab"
+                    layoutId="activeTabBottom"
                     className="absolute -bottom-1 w-8 h-0.5 bg-[var(--color-brand-primary)] rounded-full"
                     transition={{ type: 'spring', stiffness: 380, damping: 30 }}
                   />