fix(ui): stabilize infinite-stream callbacks + suppress empty-state flash

- useInfiniteStream: wrap fetchNextPage and refresh in useCallback so
  InfiniteScrollArea's IntersectionObserver does not re-subscribe on
  every parent render.
- InfiniteScrollArea: do not render 'End of stream' until at least one
  item has loaded and the initial query has settled (was flashing on
  mount before first fetch).
- AgentHealth: pass isLoading + hasItems to both InfiniteScrollArea
  wrappers.

ui/src/pages/AgentHealth/AgentHealth.tsx

@@ -957,6 +957,8 @@ export default function AgentHealth() {
             onTopVisibilityChange={setIsLogAtTop}
             isFetchingNextPage={logStream.isFetchingNextPage}
             hasNextPage={logStream.hasNextPage}
+            isLoading={logStream.isLoading}
+            hasItems={logStream.items.length > 0}
             maxHeight={360}
           >
             {filteredLogs.length > 0 ? (
@@ -993,6 +995,8 @@ export default function AgentHealth() {
             onTopVisibilityChange={setIsTimelineAtTop}
             isFetchingNextPage={eventStream.isFetchingNextPage}
             hasNextPage={eventStream.hasNextPage}
+            isLoading={eventStream.isLoading}
+            hasItems={eventStream.items.length > 0}
             maxHeight={360}
           >
             {feedEvents.length > 0 ? (