feat: add Back to Top button fixed to bottom-right corner

frontend/src/components/BackToTop.jsx

@@ -0,0 +1,41 @@
+import { useEffect, useState } from "react";
+
+export default function BackToTop() {
+  const [visible, setVisible] = useState(false);
+
+  useEffect(() => {
+    const handleScroll = () => setVisible(window.scrollY > 300);
+    handleScroll();
+    window.addEventListener("scroll", handleScroll);
+    return () => window.removeEventListener("scroll", handleScroll);
+  }, []);
+
+  const scrollToTop = () => window.scrollTo({ top: 0, behavior: "smooth" });
+
+  if (!visible) return null;
+
+  return (
+    <button
+      onClick={scrollToTop}
+      aria-label="Back to top"
+      style={{
+        position: "fixed",
+        bottom: "2rem",
+        right: "2rem",
+        zIndex: 999,
+        backgroundColor: "#C8F135", 
+        color: "black",
+        border: "none",
+        borderRadius: "50%",
+        width: "48px",
+        height: "48px",
+        fontSize: "1.4rem",
+        cursor: "pointer",
+        boxShadow: "0 4px 12px rgba(0,0,0,0.2)",
+        transition: "opacity 0.3s ease",
+      }}
+    >
+      ↑
+    </button>
+  );
+}

frontend/src/components/Landing/LandingPage.jsx

@@ -1,3 +1,4 @@
+import BackToTop from "../BackToTop";
 import React, { useEffect } from 'react';
 import { useNavigate } from 'react-router-dom';
 import { useAuth } from '../../contexts/AuthContext';
@@ -33,6 +34,7 @@ const LandingPage = () => {
       <Pricing />
       <FAQ />
       <Footer />
+      <BackToTop />
     </div>
   );
 };