feat: 优化全局样式与组件视觉升级

- 更新 globals.css 全局样式
- 优化 not-found 404 页面
- 升级 footer, mega-dropdown, mobile-tab-bar 布局组件
- 改进 challenge, cta, hero 等核心 section 组件
- 优化 challenge-card, ink-glow-card, product-card 等 UI 组件
- 更新产品常量配置

src/components/layout/mega-dropdown.tsx

@@ -74,10 +74,15 @@ export function MegaDropdown({ label, items, isOpen, onToggle, onOpen, onClose }
                 <StaticLink
                   key={item.id}
                   href={item.href}
-                  className="block p-4 rounded-lg border-l-[3px] border-l-[var(--color-brand-primary)] hover:bg-[var(--color-bg-secondary)] transition-colors duration-200"
+                  className="block p-4 rounded-lg border border-transparent hover:border-[var(--color-brand-primary)]/10 hover:bg-[var(--color-brand-primary-bg)] transition-all duration-200 group"
                 >
-                  <div className="text-sm font-semibold text-[var(--color-text-primary)]">{item.title}</div>
-                  <div className="text-xs text-[var(--color-text-placeholder)] mt-1.5 leading-relaxed">{item.description}</div>
+                  <div className="flex items-center gap-3">
+                    <div className="w-1 h-8 rounded-full bg-[var(--color-border-primary)] group-hover:bg-[var(--color-brand-primary)] transition-colors duration-200 shrink-0" />
+                    <div className="flex-1 min-w-0">
+                      <div className="text-sm font-semibold text-[var(--color-text-primary)]">{item.title}</div>
+                      <div className="text-xs text-[var(--text-placeholder)] mt-1.5 leading-relaxed">{item.description}</div>
+                    </div>
+                  </div>
                 </StaticLink>
               ))}
               </div>