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/add-to-cart/external.html.twig

@@ -0,0 +1,32 @@
+{#
+ # External/Affiliate Product Add to Cart (Bootstrap 5 Override)
+ #
+ # Renders a link button to the external product URL.
+ #
+ # Expected context:
+ #   product_url - External product URL
+ #   button_text - Button label text
+ #
+ # WooCommerce PHP equivalent: single-product/add-to-cart/external.php
+ #
+ # @package WcBootstrap
+ # @since 0.1.0
+ #}
+
+{{ do_action('woocommerce_before_add_to_cart_form') }}
+
+<form class="cart" action="{{ product_url|default('#')|esc_url }}" method="get">
+    {{ do_action('woocommerce_before_add_to_cart_button') }}
+
+    <a href="{{ product_url|default('#')|esc_url }}"
+       class="btn btn-outline-primary btn-lg single_add_to_cart_button"
+       target="_blank"
+       rel="noopener noreferrer nofollow">
+        {{ button_text|default(__('Buy product'))|esc_html }}
+        <i class="bi bi-box-arrow-up-right ms-2" aria-hidden="true"></i>
+    </a>
+
+    {{ do_action('woocommerce_after_add_to_cart_button') }}
+</form>
+
+{{ do_action('woocommerce_after_add_to_cart_form') }}