refactor(frontend): 报头抽组件 Masthead + 回到顶部抽组件 BackTop

- Masthead:统一 HomePage/StandingsLayout 双份复制实现,当前版面
  印报红高亮 + aria-current;⚙ 字符改线条 SVG;管理类入口加 title
  提示需登录,访客不再被无声踢到登录页
- BackTop:两页各一份的浮动按钮收敛为共享组件,去 rounded-full/
  shadow-lg,与全站方角无阴影语言对齐

frontend/src/components/BackTop.tsx

@@ -0,0 +1,30 @@
+/**
+ * 回到顶部浮动按钮(前台两页共用,此前 Matches/Standings 各复制一份)。
+ * 方角纸片风:去掉早期版本的 rounded-full + shadow-lg,与全站方角
+ * 无阴影语言对齐;出现/隐藏仅动画 transform 与 opacity。
+ */
+import { useEffect, useState } from 'react'
+
+export default function BackTop({ threshold = 300 }: { threshold?: number }) {
+  const [show, setShow] = useState(false)
+
+  useEffect(() => {
+    const handleScroll = () => setShow(window.scrollY > threshold)
+    window.addEventListener('scroll', handleScroll, { passive: true })
+    return () => window.removeEventListener('scroll', handleScroll)
+  }, [threshold])
+
+  return (
+    <button
+      onClick={() => window.scrollTo({ top: 0, behavior: 'smooth' })}
+      className={`fixed bottom-6 right-6 z-40 flex h-10 w-10 items-center justify-center border border-ink-300 bg-paper-50 text-ink-600 transition-all duration-300 hover:border-ink-900 hover:bg-ink-900 hover:text-paper-50 ${
+        show ? 'translate-y-0 opacity-100' : 'pointer-events-none translate-y-4 opacity-0'
+      }`}
+      aria-label="回到顶部"
+    >
+      <svg className="h-4 w-4" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
+        <path strokeLinecap="round" strokeLinejoin="round" d="M5 15l7-7 7 7" />
+      </svg>
+    </button>
+  )
+}