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/hooks/useInfiniteStream.ts

@@ -1,5 +1,5 @@
 import { useInfiniteQuery, useQueryClient } from '@tanstack/react-query';
-import { useMemo } from 'react';
+import { useCallback, useMemo } from 'react';
 
 export interface StreamPage<T> {
   data: T[];
@@ -52,14 +52,20 @@ export function useInfiniteStream<T>(args: UseInfiniteStreamArgs<T>): UseInfinit
     [query.data],
   );
 
+  const fetchNextPage = useCallback(() => {
+    if (query.hasNextPage && !query.isFetchingNextPage) query.fetchNextPage();
+  }, [query.hasNextPage, query.isFetchingNextPage, query.fetchNextPage]);
+
+  const refresh = useCallback(() => {
+    queryClient.invalidateQueries({ queryKey: [...queryKey] });
+  }, [queryClient, queryKey]);
+
   return {
     items,
-    fetchNextPage: () => {
-      if (query.hasNextPage && !query.isFetchingNextPage) query.fetchNextPage();
-    },
+    fetchNextPage,
     hasNextPage: !!query.hasNextPage,
     isFetchingNextPage: query.isFetchingNextPage,
     isLoading: query.isLoading,
-    refresh: () => queryClient.invalidateQueries({ queryKey: [...queryKey] }),
+    refresh,
   };
 }