feat(checkout): implement hierarchical grouping on rewards order confirmation page
Implements correct grouping by delivery option and target address on the
rewards order confirmation page (Prämien-Abschlussseite).
Changes:
- Add hierarchical grouping: primary by delivery type, secondary by branch
- Show branch name only when multiple branches exist within same delivery type
- Remove duplicate "Abholfiliale" section from addresses component
- Fix undefined shoppingCartItem error by providing fallback with DisplayOrderItem features
- Fix partial order creation error handling in checkout orchestrator
Implementation:
- New helpers: groupDisplayOrdersByDeliveryType, groupDisplayOrdersByBranch
- Updated reward-order-confirmation component with groupedOrders computed signal
- Added comprehensive unit tests (15 new tests, all passing)
- Graceful error handling for backend responses with partial order creation
Bug Fixes:
- Prevent undefined features error when shopping cart item not found
- Extract orders from HTTP error responses when backend returns warnings
- Add German documentation for error handling with TODO for user feedback
Related to: #5397
Related work items: #5397
