Add full cart functionality with API integration:
- Created Cart and OrderLineItem models with robust JSON parsing
- Implemented cart API methods (getOrCreateCart, setLineItem, getCart, submitOrder)
- Added cart state management to AppState with item count tracking
- Built cart view screen with item display, quantity editing, and removal
- Added cart badge to menu screen showing item count
- Implemented real add-to-cart logic with recursive modifier handling
- Added category name display in menu browsing
- Fixed API response case sensitivity (ORDER/ORDERLINEITEMS)
- Enhanced MenuItem model with categoryName field
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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
