fix: use overflow-x-clip instead of overflow-x-hidden to preserve sticky positioning

overflow-x-hidden causes overflow-y to compute to auto per CSS spec,
creating a scroll container that breaks position:sticky on descendant
elements. overflow-x-clip clips overflow without creating a scroll
container, allowing the changed-files header to stick correctly.

Author: cursoragent

apps/web/src/components/chat/MessagesTimeline.tsx

@@ -234,7 +234,7 @@ export const MessagesTimeline = memo(function MessagesTimeline({
   // from TimelineRowCtx, which propagates through LegendList's memo.
   const renderItem = useCallback(
     ({ item }: { item: MessagesTimelineRow }) => (
-      <div className="mx-auto w-full min-w-0 max-w-3xl overflow-x-hidden" data-timeline-root="true">
+      <div className="mx-auto w-full min-w-0 max-w-3xl overflow-x-clip" data-timeline-root="true">
         <TimelineRowContent row={item} />
       </div>
     ),