feat: scrollTop 기능 추가

src/components/ui/common/FloatBtn.tsx

@@ -5,9 +5,16 @@ import { usePathname } from 'next/navigation'
 export default function FloatBtn() {
   const pathname = usePathname()
 
+  const scrollToTop = () => {
+    window.scrollTo({
+      top: 0,
+      behavior: 'smooth',
+    })
+  }
+
   if (pathname === '/login' || pathname === '/') {
     return null
   }
 
-  return <button className="top-btn"></button>
+  return <button className="top-btn" onClick={scrollToTop}></button>
 }