fix: page indicator stuck on page 1 due to stale closure

The IntersectionObserver callback was using currentPage from closure to
initialize mostVisiblePage, but this was always stale (stuck at 0).

Changed to start mostVisiblePage at 0 instead of currentPage, so it
correctly tracks the most visible page across all entries without
relying on stale state.

🤖 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

@@ -172,7 +172,7 @@ function PreviewContent({ formState, initialData, isScrolling = false }: Workshe
     const observer = new IntersectionObserver(
       (entries) => {
         // Find the most visible page among all entries
-        let mostVisiblePage = currentPage
+        let mostVisiblePage = 0
         let maxRatio = 0
 
         entries.forEach((entry) => {