Fix duplicate page tokens when navigating jelly-pagination window

[Copilot]

Pagination intermittently rendered duplicate numeric buttons when crossing window boundaries (e.g., moving from page 1 to 6 could show 5 twice). This change makes page-window construction uniqueness-safe and adds a focused regression covering the reported path.

• Pagination window generation

  • Reworked pagesToShow() to build candidate pages via a Set (1, total, current ± 1) and then render sorted pages.
  • Ellipsis insertion now depends on actual numeric gaps between adjacent rendered pages, preventing duplicate tokens.

• Regression coverage

  • Added a pagination test that advances from page 1 to page 6 via next-button clicks and asserts:
    • page state is correct (page === 6)
    • numeric page labels are unique
    • page 5 appears exactly once

const pages = new Set<number>([1, total]);
for (let p = cur - 1; p <= cur + 1; p++) {
  if (p >= 1 && p <= total) pages.add(p);
}
for (const p of [...pages].sort((a, b) => a - b)) {
  if (previous !== null && p - previous > 1) out.push('…');
  out.push(p);
  previous = p;
}

• Fixes <jelly-pagination> from page 1 click to 6, 5 will show twice #1