feat(shop): category filter bar + sort/load-more 1:1 UX

Port zibll hierarchical cat tabs, multi-metric sort with price toggle,
skeleton/empty states, and REST archive payload (header/html/paginate)
while reusing product-card template for AJAX HTML.

templates/parts/shop/product-card.php

@@ -34,7 +34,7 @@ $image = (string) ($product['image'] ?? '');
 $excerpt = (string) ($product['excerpt'] ?? '');
 $price = (float) ($product['price'] ?? 0);
 ?>
-<article class="aksrt-product-card">
+<article class="aksrt-product-card product-item posts-item card ajax-item">
 	<a href="<?php echo esc_url($permalink !== '' ? $permalink : '#'); ?>" class="aksrt-product-card__link">
 		<?php if ($image !== '') : ?>
 			<div class="aksrt-product-card__image">