fix(ui): suppress double scrollbar in log + timeline panels

LogViewer and EventFeed each apply overflow-y:auto to their root
container, which produced a nested scrollbar inside the
InfiniteScrollArea that also scrolls. A flatScroll override class
flattens the DS component so the outer InfiniteScrollArea owns the
single scrollbar — matching the IntersectionObserver sentinels that
drive infinite-load.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/pages/AgentInstance/AgentInstance.tsx

@@ -472,7 +472,7 @@ export default function AgentInstance() {
             maxHeight={360}
           >
             {filteredLogs.length > 0 ? (
-              <LogViewer entries={filteredLogs} />
+              <LogViewer entries={filteredLogs} className={logStyles.flatScroll} />
             ) : (
               <div className={logStyles.logEmpty}>
                 {logSearch || logLevels.size > 0 || logSources.size > 0
@@ -510,7 +510,7 @@ export default function AgentInstance() {
             maxHeight={360}
           >
             {feedEvents.length > 0 ? (
-              <EventFeed events={feedEvents} />
+              <EventFeed events={feedEvents} className={logStyles.flatScroll} />
             ) : (
               <div className={logStyles.logEmpty}>
                 {eventStream.isLoading ? 'Loading events…' : 'No events in the selected time range.'}