Move scroll-to-bottom button to issue detail and run pages

Removed the scroll-to-bottom button from IssuesList (wrong location)
and created a shared ScrollToBottom component. Added it to IssueDetail
and RunDetail pages. On mobile, the button sits above the bottom nav
to avoid overlap.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/components/ScrollToBottom.tsx

@@ -0,0 +1,40 @@
+import { useCallback, useEffect, useState } from "react";
+import { ArrowDown } from "lucide-react";
+
+/**
+ * Floating scroll-to-bottom button that appears when the user is far from the
+ * bottom of the `#main-content` scroll container. Hides when within 300px of
+ * the bottom. Positioned to avoid the mobile bottom nav.
+ */
+export function ScrollToBottom() {
+  const [visible, setVisible] = useState(false);
+
+  useEffect(() => {
+    const el = document.getElementById("main-content");
+    if (!el) return;
+    const check = () => {
+      const distance = el.scrollHeight - el.scrollTop - el.clientHeight;
+      setVisible(distance > 300);
+    };
+    check();
+    el.addEventListener("scroll", check, { passive: true });
+    return () => el.removeEventListener("scroll", check);
+  }, []);
+
+  const scroll = useCallback(() => {
+    const el = document.getElementById("main-content");
+    if (el) el.scrollTo({ top: el.scrollHeight, behavior: "smooth" });
+  }, []);
+
+  if (!visible) return null;
+
+  return (
+    <button
+      onClick={scroll}
+      className="fixed bottom-[calc(1.5rem+5rem+env(safe-area-inset-bottom))] right-6 z-40 flex h-9 w-9 items-center justify-center rounded-full border border-border bg-background shadow-md hover:bg-accent transition-colors md:bottom-6"
+      aria-label="Scroll to bottom"
+    >
+      <ArrowDown className="h-4 w-4" />
+    </button>
+  );
+}