Implement shopping cart page and interactions

[yortch]

Summary

• implement cart page UI aligned to docs/design/cart.png with cart table and order summary
• add cart state management via CartContext with localStorage persistence and coupon handling
• wire cart route and navigation links, and connect Products add-to-cart flow to shared cart state
• add frontend cart calculation and interaction tests using Vitest + Testing Library

Fixes #15

Validation

• npm run test --workspace=frontend
• npm run build --workspace=frontend

Notes

• excludes unrelated local change in api/src/routes/order.ts