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>

PLAN.md

@@ -573,30 +573,30 @@ Track completion per file. Mark with `[x]` when done.
 
 ### Phase 4 -- Cart
 
-- [ ] `cart/cart.html.twig`
-- [ ] `cart/cart-empty.html.twig`
-- [ ] `cart/cart-item-data.html.twig`
-- [ ] `cart/cart-totals.html.twig`
-- [ ] `cart/cart-shipping.html.twig`
-- [ ] `cart/cross-sells.html.twig`
-- [ ] `cart/mini-cart.html.twig`
-- [ ] `cart/proceed-to-checkout-button.html.twig`
-- [ ] `cart/shipping-calculator.html.twig`
+- [x] `cart/cart.html.twig`
+- [x] `cart/cart-empty.html.twig`
+- [x] `cart/cart-item-data.html.twig`
+- [x] `cart/cart-totals.html.twig`
+- [x] `cart/cart-shipping.html.twig`
+- [x] `cart/cross-sells.html.twig`
+- [x] `cart/mini-cart.html.twig`
+- [x] `cart/proceed-to-checkout-button.html.twig`
+- [x] `cart/shipping-calculator.html.twig`
 
 ### Phase 5 -- Checkout
 
-- [ ] `checkout/form-checkout.html.twig`
-- [ ] `checkout/form-billing.html.twig`
-- [ ] `checkout/form-shipping.html.twig`
-- [ ] `checkout/form-coupon.html.twig`
-- [ ] `checkout/form-login.html.twig`
-- [ ] `checkout/review-order.html.twig`
-- [ ] `checkout/payment.html.twig`
-- [ ] `checkout/payment-method.html.twig`
-- [ ] `checkout/terms.html.twig`
-- [ ] `checkout/thankyou.html.twig`
-- [ ] `checkout/order-received.html.twig`
-- [ ] `checkout/cart-errors.html.twig`
+- [x] `checkout/form-checkout.html.twig`
+- [x] `checkout/form-billing.html.twig`
+- [x] `checkout/form-shipping.html.twig`
+- [x] `checkout/form-coupon.html.twig`
+- [x] `checkout/form-login.html.twig`
+- [x] `checkout/review-order.html.twig`
+- [x] `checkout/payment.html.twig`
+- [x] `checkout/payment-method.html.twig`
+- [x] `checkout/terms.html.twig`
+- [x] `checkout/thankyou.html.twig`
+- [x] `checkout/order-received.html.twig`
+- [x] `checkout/cart-errors.html.twig`
 
 ### Phase 6 -- My Account