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/cart-empty.html.twig

@@ -0,0 +1,23 @@
+{#
+ # Empty Cart (Bootstrap 5 Override)
+ #
+ # Displayed when the cart has no items.
+ #
+ # WooCommerce PHP equivalent: cart/cart-empty.php
+ #
+ # @package WcBootstrap
+ # @since 0.1.0
+ #}
+
+{{ do_action('woocommerce_cart_is_empty') }}
+
+<div class="text-center py-5">
+    <i class="bi bi-cart-x display-1 text-body-secondary mb-4" aria-hidden="true"></i>
+    <p class="lead text-body-secondary mb-4">{{ __('Your cart is currently empty.') }}</p>
+
+    {% if wc_get_page_id('shop') > 0 %}
+        <a class="btn btn-primary btn-lg" href="{{ wc_get_page_permalink('shop')|esc_url }}">
+            {{ __('Return to shop') }}
+        </a>
+    {% endif %}
+</div>