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/components/InfiniteScrollArea.tsx

@@ -6,6 +6,8 @@ export interface InfiniteScrollAreaProps {
   onTopVisibilityChange?: (atTop: boolean) => void;
   isFetchingNextPage: boolean;
   hasNextPage: boolean;
+  isLoading?: boolean;
+  hasItems?: boolean;
   maxHeight?: number | string;
   children: ReactNode;
   /** Optional caller-owned scroll container ref (e.g. for scroll-to-top on refresh). */
@@ -18,6 +20,8 @@ export function InfiniteScrollArea({
   onTopVisibilityChange,
   isFetchingNextPage,
   hasNextPage,
+  isLoading = false,
+  hasItems = true,
   maxHeight = 360,
   children,
   scrollRef,
@@ -68,7 +72,9 @@ export function InfiniteScrollArea({
       {children}
       <div ref={bottomSentinel} className={styles.sentinel} aria-hidden="true" />
       {isFetchingNextPage && <div className={styles.loadingMore}>Loading more…</div>}
-      {!hasNextPage && <div className={styles.endOfStream}>End of stream</div>}
+      {!hasNextPage && !isLoading && hasItems && (
+        <div className={styles.endOfStream}>End of stream</div>
+      )}
     </div>
   );
 }