fix(board): show total count in Done column and infinite scroll (#501)

* fix(board): show total count in Done column header and auto-load on scroll

- Column header now shows server-side doneTotal instead of loaded count
- Replace "Load more" button with IntersectionObserver sentinel for
  infinite scroll in the Done column

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(board): move sentinel below imports and stabilize observer

- Move InfiniteScrollSentinel after all import statements
- Use callback ref to avoid recreating IntersectionObserver on every render

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(board): add optional chaining for IntersectionObserver entry

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/features/issues/components/board-view.tsx

@@ -43,7 +43,7 @@ function InfiniteScrollSentinel({ onVisible, loading }: { onVisible: () => void;
     const node = sentinelRef.current;
     if (!node) return;
     const observer = new IntersectionObserver(
-      ([entry]) => { if (entry.isIntersecting) onVisibleRef.current(); },
+      ([entry]) => { if (entry?.isIntersecting) onVisibleRef.current(); },
       { rootMargin: "100px" },
     );
     observer.observe(node);