feat(ui): 重构核心 UI 组件库，新增 shadcn/ui 组件

- 重构 Button、Card、Badge、Input、Textarea 等基础组件
- 新增 Accordion、Alert、Dialog、Dropdown、Form 等 shadcn/ui 组件
- 新增 AnimatedCounter、StatsShowcase、MetricCard 等数据展示组件
- 新增 ScrollReveal 滚动动画组件
- 重构 Toast 通知系统与 Tooltip 提示组件
- 更新设计令牌系统，对齐新品牌视觉

src/components/ui/back-to-top.tsx

@@ -35,13 +35,10 @@ export function BackToTop() {
           exit={shouldReduceMotion ? {} : { opacity: 0, y: 20, scale: 0.8 }}
           transition={{ duration: 0.2, ease: 'easeOut' }}
           onClick={scrollToTop}
-          className="fixed right-4 bottom-20 md:bottom-8 md:right-8 z-40 p-3 bg-[var(--color-brand-primary)] text-white rounded-full shadow-lg hover:bg-[var(--color-brand-primary-hover)] hover:shadow-xl transition-all duration-200 focus:outline-none focus:ring-2 focus:ring-[var(--color-brand-primary)] focus:ring-offset-2"
+          className="fixed right-4 bottom-20 md:bottom-8 md:right-8 z-40 p-3 bg-brand text-white hover:bg-brand-hover transition-all duration-300 ease-out focus:outline-none focus:ring-2 focus:ring-brand focus:ring-offset-2 focus:ring-offset-white"
           aria-label="返回顶部"
           title="返回顶部"
-          style={{
-            boxShadow: '0 4px 14px rgba(var(--color-brand-primary-rgb), 0.4)',
-          }}
-          whileHover={shouldReduceMotion ? {} : { scale: 1.1 }}
+          whileHover={shouldReduceMotion ? {} : { scale: 1.05 }}
           whileTap={shouldReduceMotion ? {} : { scale: 0.95 }}
         >
           <ArrowUp className="w-6 h-6" />