feat(ui): add InfiniteScrollArea component

Scrollable container with top/bottom IntersectionObserver sentinels.
Fires onTopVisibilityChange when the top is fully in view and
onEndReached when the bottom is within 100px. Used by infinite log
and event streams.

ui/src/components/InfiniteScrollArea.module.css

@@ -0,0 +1,22 @@
+.scrollArea {
+  overflow-y: auto;
+  position: relative;
+}
+
+.sentinel {
+  height: 1px;
+  width: 100%;
+  pointer-events: none;
+}
+
+.loadingMore,
+.endOfStream {
+  padding: 8px 12px;
+  font-size: 11px;
+  color: var(--text-muted);
+  text-align: center;
+}
+
+.endOfStream {
+  opacity: 0.5;
+}

ui/src/components/InfiniteScrollArea.tsx

@@ -0,0 +1,74 @@
+import { useEffect, useRef, type ReactNode, type RefObject } from 'react';
+import styles from './InfiniteScrollArea.module.css';
+
+export interface InfiniteScrollAreaProps {
+  onEndReached: () => void;
+  onTopVisibilityChange?: (atTop: boolean) => void;
+  isFetchingNextPage: boolean;
+  hasNextPage: boolean;
+  maxHeight?: number | string;
+  children: ReactNode;
+  /** Optional caller-owned scroll container ref (e.g. for scroll-to-top on refresh). */
+  scrollRef?: RefObject<HTMLDivElement | null>;
+  className?: string;
+}
+
+export function InfiniteScrollArea({
+  onEndReached,
+  onTopVisibilityChange,
+  isFetchingNextPage,
+  hasNextPage,
+  maxHeight = 360,
+  children,
+  scrollRef,
+  className,
+}: InfiniteScrollAreaProps) {
+  const internalRef = useRef<HTMLDivElement | null>(null);
+  const containerRef = scrollRef ?? internalRef;
+  const topSentinel = useRef<HTMLDivElement | null>(null);
+  const bottomSentinel = useRef<HTMLDivElement | null>(null);
+
+  useEffect(() => {
+    if (!onTopVisibilityChange) return;
+    const root = containerRef.current;
+    const target = topSentinel.current;
+    if (!root || !target) return;
+    const obs = new IntersectionObserver(
+      (entries) => {
+        for (const e of entries) onTopVisibilityChange(e.isIntersecting);
+      },
+      { root, threshold: 1.0 },
+    );
+    obs.observe(target);
+    return () => obs.disconnect();
+  }, [containerRef, onTopVisibilityChange]);
+
+  useEffect(() => {
+    if (!hasNextPage) return;
+    const root = containerRef.current;
+    const target = bottomSentinel.current;
+    if (!root || !target) return;
+    const obs = new IntersectionObserver(
+      (entries) => {
+        for (const e of entries) if (e.isIntersecting) onEndReached();
+      },
+      { root, rootMargin: '100px', threshold: 0 },
+    );
+    obs.observe(target);
+    return () => obs.disconnect();
+  }, [containerRef, onEndReached, hasNextPage]);
+
+  return (
+    <div
+      ref={containerRef}
+      className={`${styles.scrollArea}${className ? ` ${className}` : ''}`}
+      style={{ maxHeight }}
+    >
+      <div ref={topSentinel} className={styles.sentinel} aria-hidden="true" />
+      {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>}
+    </div>
+  );
+}