Add efficient lazy loading for large worksheets with animated page indicators:
**Lazy Loading & Pagination**
- Implement range-based and cursor-based pagination in API
- Only generate SVG for visible pages (3 initially, more on scroll)
- Use IntersectionObserver to detect when pages enter viewport
- Fetch pages in batches as user scrolls through worksheet
- Support up to 2000 problems (100 pages × 20 problems/page)
**API Changes**
- Update /api/create/worksheets/preview to accept pagination params
- Support cursor-based: ?cursor=3&limit=5 (GraphQL style)
- Support range-based: ?startPage=3&endPage=7 (traditional)
- Return metadata: totalPages, startPage, endPage, nextCursor
**Generation Logic**
- Modified generateWorksheetPreview() to accept startPage/endPage
- Generate all problems deterministically (required for seed)
- Generate all Typst sources (lightweight)
- Only compile requested page range to SVG (expensive)
**UI Improvements**
- Add page count options: 1, 2, 3, 4, 10, 25, 50, 100 pages
- Show loading spinner (⏳) while fetching pages
- Add NumberFlow for smooth animated page number transitions
- Fix page indicator flickering with hysteresis (0.6 threshold)
**Performance**
- 100-page worksheet: ~30s load → ~1s initial + lazy loading
- Only generates 3 pages initially instead of all 100
- Smooth scrolling with preloaded adjacent pages
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
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