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/pages/IssueDetail.tsx

@@ -18,6 +18,7 @@ import { CommentThread } from "../components/CommentThread";
 import { IssueProperties } from "../components/IssueProperties";
 import { LiveRunWidget } from "../components/LiveRunWidget";
 import type { MentionOption } from "../components/MarkdownEditor";
+import { ScrollToBottom } from "../components/ScrollToBottom";
 import { StatusIcon } from "../components/StatusIcon";
 import { PriorityIcon } from "../components/PriorityIcon";
 import { StatusBadge } from "../components/StatusBadge";
@@ -926,6 +927,7 @@ export function IssueDetail() {
           </ScrollArea>
         </SheetContent>
       </Sheet>
+      <ScrollToBottom />
     </div>
   );
 }