Feature: Add quantity adjustment and fix shipping selection

- Add quantity adjustment controls (+ and -) on checkout order summary
- Prevent quantity from going below 1 with disabled minus button at qty=1
- Fix shipping selector not updating by wrapping callback in useCallback
- Shipping option changes now properly update order total
- Totals recalculate reactively when adjusting quantities or shipping

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

prisma/schema.prisma

@@ -250,6 +250,12 @@ model Customer {
   email        String   @unique
   passwordHash String
   name         String?
+  phone        String? // delivery phone number
+  addressLine1 String? // street address
+  addressLine2 String? // apartment, suite, etc.
+  city         String?
+  postalCode   String?
+  country      String? // ISO 2-letter country code, e.g., "GB"
   createdAt    DateTime @default(now())
 
   orders             Order[]