function create_full_category_grid_shortcode() {
    // --- CONFIGURATION ---
    $taxonomy_slug = 'research-category';
    $acf_field_name = 'research_category_featured_image';
    // --- END CONFIGURATION ---

    $parent_terms = get_terms(array(
        'taxonomy'   => $taxonomy_slug,
        'parent'     => 0,
        'hide_empty' => false,
    ));

    if (empty($parent_terms) || is_wp_error($parent_terms)) {
        return '<p>No parent categories found.</p>';
    }

    $full_output = '';

    foreach ($parent_terms as $parent_term) {
        $full_output .= '<div class="research-parent-category-section">';
        $full_output .= '<h2 class="research-parent-category-title">' . esc_html($parent_term->name) . '</h2>';

        $child_terms = get_terms(array(
            'taxonomy'   => $taxonomy_slug,
            'parent'     => $parent_term->term_id,
            'hide_empty' => false,
        ));

        if (!empty($child_terms) && !is_wp_error($child_terms)) {
            $full_output .= '<div class="research-subcategory-grid-container">';

            foreach ($child_terms as $child_term) {
                $term_link = get_term_link($child_term);
                $term_name = $child_term->name;
                $image_url = '';

                $image_field = get_field($acf_field_name, $child_term);
                if ($image_field) {
                    if (is_array($image_field)) {
                        $image_url = $image_field['url'];
                    } elseif (is_numeric($image_field)) {
                        $image_url = wp_get_attachment_image_url($image_field, 'large');
                    } else {
                        $image_url = $image_field;
                    }
                }

                $full_output .= '<a href="' . esc_url($term_link) . '" class="research-subcategory-item">';
                if ($image_url) {
                    $full_output .= '<img src="' . esc_url($image_url) . '" alt="' . esc_attr($term_name) . '">';
                }
                $full_output .= '<div class="research-subcategory-title-box">';
                $full_output .= '<h3 class="research-subcategory-title lead">' . esc_html($term_name) . '</h3>';
                $full_output .= '</div>';
                $full_output .= '</a>';
            }

            $full_output .= '</div>';
        }

        $full_output .= '</div>';
    }

    return $full_output;
}

add_shortcode('full_category_grid', 'create_full_category_grid_shortcode');