feat: 添加“返回顶部”组件，提升用户导航体验

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

@@ -0,0 +1,46 @@
+import { ArrowUp } from "lucide-react";
+import { Button } from "./button";
+import { useEffect, useState } from "react";
+
+export default function BackToTop() {
+  const [isVisible, setIsVisible] = useState(false);
+
+  useEffect(() => {
+    const toggleVisibility = () => {
+      if (window.scrollY > 300) {
+        setIsVisible(true);
+      } else {
+        setIsVisible(false);
+      }
+    };
+
+    window.addEventListener("scroll", toggleVisibility);
+    return () => window.removeEventListener("scroll", toggleVisibility);
+  }, []);
+
+  const scrollToTop = () => {
+    window.scrollTo({
+      top: 0,
+      behavior: "smooth",
+    });
+  };
+
+  return (
+    <div
+      className={`fixed bottom-8 right-8 z-50 transition-all duration-300 ${
+        isVisible
+          ? "translate-y-0 opacity-100"
+          : "translate-y-4 opacity-0 pointer-events-none"
+      }`}
+    >
+      <Button
+        size="icon"
+        onClick={scrollToTop}
+        className="rounded-full shadow-lg bg-primary/90 backdrop-blur-sm border border-border/50 hover:bg-primary hover:shadow-xl hover:-translate-y-1 transition-all h-10 w-10"
+      >
+        <ArrowUp className="h-5 w-5 text-primary-foreground" />
+        <span className="sr-only">Back to top</span>
+      </Button>
+    </div>
+  );
+}