feat(ui): 全站画布氛围层、侧栏 sticky 对齐与长页回顶

用顶栏/主栏高度令牌消除 sticky 首滚空走，面包屑叠到中栏上方，公开页增加回顶浮钮。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/components/BackToTop.tsx

@@ -0,0 +1,39 @@
+"use client";
+
+import { useEffect, useState } from "react";
+import { ArrowUp } from "lucide-react";
+import { scrollBehavior } from "@/lib/motion";
+
+const SHOW_AFTER = 400;
+
+/**
+ * 公开站长页回顶：滚过阈值后右下角浮现。
+ * 短页自然不出现；帖子移动端底栏时抬高，避免叠住操作条。
+ */
+export default function BackToTop({
+  liftForMobileBar = false,
+}: {
+  liftForMobileBar?: boolean;
+}) {
+  const [visible, setVisible] = useState(false);
+
+  useEffect(() => {
+    const onScroll = () => setVisible(window.scrollY > SHOW_AFTER);
+    onScroll();
+    window.addEventListener("scroll", onScroll, { passive: true });
+    return () => window.removeEventListener("scroll", onScroll);
+  }, []);
+
+  if (!visible) return null;
+
+  return (
+    <button
+      type="button"
+      className={`j13-back-top j13-toast-in${liftForMobileBar ? " is-lifted" : ""}`}
+      onClick={() => window.scrollTo({ top: 0, behavior: scrollBehavior() })}
+      aria-label="回到顶部"
+    >
+      <ArrowUp size={18} strokeWidth={2.25} aria-hidden />
+    </button>
+  );
+}