Build ShippingAddressSection (address + 50-state dropdown + ZIP)

## Goal
Build the shipping address section with street, city, a 50-state dropdown, and ZIP. Validate with Zod (including a ZIP regex).

## Acceptance criteria
- [ ] State dropdown uses the shared `Select` with all 50 US states
- [ ] ZIP validated against `^\d{5}(-\d{4})?$`
- [ ] All fields required
- [ ] Per-field error messages

Start by locating the existing shared Select component and the form entry point for ShippingAddressSection. Implement the street, city, state, and ZIP fields with required per-field errors, all 50 states, and the specified ZIP regex; done means every acceptance criterion is satisfied.

Written by the indexing model from the issue text.