a11y: listing category filter h5 breaks heading-order #14384

Reported in #14373.

Repro: https://github.com/alexpghayes/quarto-accessibility-reprex

axe rule: heading-order · impact: moderate · tag: best-practice
Target: <h5 class="quarto-listing-category-title">Categories</h5> on listing pages with categories: true.

The listing template renders the category filter title as <h5> regardless of the surrounding heading context. On a listing page whose own title is <h1> with no intermediate h2–h4, jumping to h5 breaks the heading hierarchy. Should use <h2> (or match the current heading depth) so screen reader users get consistent navigation.

Part of #8706.