Implement Phase 4 & 5: cart and checkout templates (Bootstrap 5, HPOS)

Phase 4 - Cart (9 templates):
- cart: 8+4 column layout, table-responsive items, coupon input-group
- cart-empty: centered empty state with cart-x icon
- cart-item-data: inline dl for variation details
- cart-totals: card with list-group-flush rows, sticky sidebar
- cart-shipping: form-check radio per shipping method
- cross-sells: product loop grid section
- mini-cart: offcanvas-compatible item list with remove buttons
- proceed-to-checkout-button: btn-primary btn-lg w-100
- shipping-calculator: collapsible form with form-select/form-control

Phase 5 - Checkout (12 templates):
- form-checkout: 7+5 column layout, sticky order review sidebar
- form-billing: card with field wrapper, optional account creation
- form-shipping: card with ship-to-different-address collapse toggle
- form-coupon: collapsible input-group with alert-info toggle
- form-login: collapsible login reusing global/form-login.html.twig
- review-order: card with table-sm, tfoot subtotal/shipping/total
- payment: list-group of payment gateways with radio selection
- payment-method: form-check with description collapse
- terms: form-check checkbox with T&C link
- thankyou: HPOS compatible, alert-success + order details list-group
- order-received: confirmation message
- cart-errors: alert-danger with return-to-cart button

All order data accessed via WC_Order methods (HPOS compatible).
CSS additions: cart thumbnail sizing, checkout form field overrides,
payment box transitions, dark mode focus states.

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

templates/cart/cross-sells.html.twig

@@ -0,0 +1,32 @@
+{#
+ # Cross-sell Products (Bootstrap 5 Override)
+ #
+ # Renders cross-sell product recommendations below the cart.
+ #
+ # Expected context:
+ #   cross_sells - Array of WC_Product objects
+ #   columns     - Number of columns
+ #   heading     - Section heading (filtered)
+ #
+ # WooCommerce PHP equivalent: cart/cross-sells.php
+ #
+ # @package WcBootstrap
+ # @since 0.1.0
+ #}
+
+{% if cross_sells is defined and cross_sells|length > 0 %}
+    <section class="cross-sells mt-5">
+        {% set heading = heading|default(__('You may be interested in&hellip;')) %}
+        {% if heading %}
+            <h2 class="h4 mb-4">{{ heading }}</h2>
+        {% endif %}
+
+        {{ woocommerce_product_loop_start() }}
+
+        {% for product in cross_sells %}
+            {% include 'content-product.html.twig' with { product: product } %}
+        {% endfor %}
+
+        {{ woocommerce_product_loop_end() }}
+    </section>
+{% endif %}