fix: remove pages from visible set when they leave viewport

The virtualization was adding pages but never removing them, causing
all pages to eventually be rendered and defeating the purpose.

Changed logic to:
1. Start with empty set for each observer callback
2. Add currently intersecting pages + adjacent pages
3. Keep pages that weren't observed in this callback (unchanged)
4. Remove pages that are observed but not intersecting

This ensures pages are removed from the visible set when scrolled away.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

apps/web/src/app/create/worksheets/components/WorksheetPreview.tsx

@@ -185,8 +185,9 @@ function PreviewContent({ formState, initialData, isScrolling = false }: Workshe
         // Update visible pages set (only when virtualizing)
         if (shouldVirtualize) {
           setVisiblePages((prev) => {
-            const next = new Set(prev)
+            const next = new Set<number>()
 
+            // Only keep pages that are currently intersecting
             entries.forEach((entry) => {
               const pageIndex = Number(entry.target.getAttribute('data-page-index'))
 
@@ -197,6 +198,16 @@ function PreviewContent({ formState, initialData, isScrolling = false }: Workshe
                 // Preload adjacent pages for smooth scrolling
                 if (pageIndex > 0) next.add(pageIndex - 1)
                 if (pageIndex < totalPages - 1) next.add(pageIndex + 1)
+              } else {
+                console.log('[VIRTUALIZATION] Page ' + pageIndex + ' is NOT intersecting - checking if should remove')
+              }
+            })
+
+            // Keep any pages from prev that weren't in entries (not observed in this callback)
+            prev.forEach((pageIndex) => {
+              const wasObserved = entries.some((entry) => Number(entry.target.getAttribute('data-page-index')) === pageIndex)
+              if (!wasObserved) {
+                next.add(pageIndex)
               }
             })