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/checkout/form-coupon.html.twig

@@ -0,0 +1,45 @@
+{#
+ # Checkout Coupon Form (Bootstrap 5 Override)
+ #
+ # Collapsible coupon code input on the checkout page.
+ #
+ # WooCommerce PHP equivalent: checkout/form-coupon.php
+ #
+ # @package WcBootstrap
+ # @since 0.1.0
+ #}
+
+{% if wc_coupons_enabled() %}
+    <div class="woocommerce-form-coupon-toggle mb-4">
+        <div class="alert alert-info d-flex align-items-center" role="status">
+            <i class="bi bi-tag me-2" aria-hidden="true"></i>
+            {{ __('Have a coupon?') }}
+            <a href="#checkoutCoupon"
+               class="ms-1 alert-link"
+               data-bs-toggle="collapse"
+               role="button"
+               aria-expanded="false"
+               aria-controls="checkoutCoupon">
+                {{ __('Click here to enter your code') }}
+            </a>
+        </div>
+    </div>
+
+    <div class="collapse mb-4" id="checkoutCoupon">
+        <form class="checkout_coupon woocommerce-form-coupon" method="post">
+            <div class="input-group">
+                <input type="text"
+                       name="coupon_code"
+                       class="form-control"
+                       id="coupon_code_checkout"
+                       placeholder="{{ __('Coupon code') }}" />
+                <button type="submit"
+                        class="btn btn-outline-secondary"
+                        name="apply_coupon"
+                        value="{{ __('Apply coupon') }}">
+                    {{ __('Apply coupon') }}
+                </button>
+            </div>
+        </form>
+    </div>
+{% endif %}