Implement Phase 3: single product page templates (Bootstrap 5)

Add 21 Twig template overrides for the single product page:

Product layout:
- product-image: gallery with thumbnail strip, img-fluid rounded
- title: h1 entry-title
- price: fs-3 fw-bold with sale del/ins markup
- short-description: lead text-body-secondary
- meta: dl row with SKU, categories, tags
- rating: Bootstrap Icon stars with half-star, review count link
- stock: badge (bg-success/bg-danger/bg-warning) per status
- sale-flash: badge bg-danger fs-6
- share: hook-only wrapper
- product-attributes: table-sm table-striped

Related/upsells:
- related, up-sells: section with product loop grid

Tabs:
- tabs: nav-tabs + tab-content with fade transitions
- description: tab-pane with prose content
- additional-information: tab-pane with attributes hook

Add to cart (4 product types + variation JS):
- simple: input-group quantity + btn-primary btn-lg
- variable: form-select per attribute + variation display
- grouped: table-borderless with quantity per child product
- external: btn-outline-primary with external link icon
- variation: Underscore.js script templates (Bootstrap-styled)
- variation-add-to-cart-button: quantity + submit with hidden fields

CSS additions: gallery thumbnail hover, variation selector spacing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/single-product/related.html.twig

@@ -0,0 +1,32 @@
+{#
+ # Related Products (Bootstrap 5 Override)
+ #
+ # Renders the related products section below the single product.
+ #
+ # Expected context:
+ #   related_products - Array of WC_Product objects
+ #   columns          - Number of columns for the grid
+ #   heading          - Section heading text (filtered)
+ #
+ # WooCommerce PHP equivalent: single-product/related.php
+ #
+ # @package WcBootstrap
+ # @since 0.1.0
+ #}
+
+{% if related_products is defined and related_products|length > 0 %}
+    <section class="related products mt-5">
+        {% set heading = heading|default(__('Related products')) %}
+        {% if heading %}
+            <h2 class="h4 mb-4">{{ heading|esc_html }}</h2>
+        {% endif %}
+
+        {{ woocommerce_product_loop_start() }}
+
+        {% for product in related_products %}
+            {% include 'content-product.html.twig' with { product: product } %}
+        {% endfor %}
+
+        {{ woocommerce_product_loop_end() }}
+    </section>
+{% endif %}