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/stock.html.twig

@@ -0,0 +1,31 @@
+{#
+ # Stock Status (Bootstrap 5 Override)
+ #
+ # Renders the product stock status as a Bootstrap badge.
+ #
+ # Expected context:
+ #   class        - Stock CSS class ('in-stock', 'out-of-stock', 'on-backorder')
+ #   availability - Stock availability text
+ #
+ # WooCommerce PHP equivalent: single-product/stock.php
+ #
+ # @package WcBootstrap
+ # @since 0.1.0
+ #}
+
+{% if availability is defined and availability %}
+    {% set badge_class = 'bg-secondary' %}
+    {% if class is defined %}
+        {% if 'in-stock' in class %}
+            {% set badge_class = 'bg-success' %}
+        {% elseif 'out-of-stock' in class %}
+            {% set badge_class = 'bg-danger' %}
+        {% elseif 'on-backorder' in class %}
+            {% set badge_class = 'bg-warning text-dark' %}
+        {% endif %}
+    {% endif %}
+
+    <p class="stock {{ class|default('')|esc_attr }} mb-3">
+        <span class="badge {{ badge_class }}">{{ availability|esc_html }}</span>
+    </p>
+{% endif %}