Feature: Royal Mail shipping integration with weight-based postage

Added:
- Weight field (weightGrams) to Product model for storing item weights
- Shipping fields to Order model (shippingCost, shippingCountry, shippingService)
- Royal Mail shipping service (royalmail-shipping.ts) with:
  * Domestic and international postage calculations
  * Integration ready for Royal Mail API (currently uses estimated rates)
  * Support for multiple shipping services
  * Clear international delivery time warnings
- Shipping quotes API endpoint (/api/checkout/shipping-quotes)
- Checkout integration to calculate and include shipping cost

Weight is stored in grams, displayed to users in kilograms.
Shipping cost is calculated based on total cart weight and destination country.

prisma/schema.prisma

@@ -41,6 +41,7 @@ model Product {
   printAreaHeightMm Int @default(280) // height of the printable area in millimeters
   referenceWidthCm Int? // width of garment/object for ruler reference (cm)
   referenceHeightCm Int? // height of garment/object for ruler reference (cm)
+  weightGrams Int? // weight in grams (used for Royal Mail postage calculation via API)
   imageUrl    String? // optional real product photo — shown instead of the illustration when set
   imageUrlBack String? // optional back-view photo, toggled alongside the front photo
   photoRecolorable Boolean @default(false) // if true, the photo is treated as a grayscale
@@ -218,12 +219,15 @@ model Order {
   stripeSessionId String?     @unique
   email           String?
   shippingAddress String? // JSON — collected by Stripe Checkout, saved once payment completes
+  shippingCountry String? // ISO 2-letter country code (e.g., 'GB', 'US') for Royal Mail shipping
   status          String      @default("PENDING") // PENDING | PAID | PRINTING | PRINTED | SHIPPED | DELIVERED | FAILED
   archived        Boolean     @default(false)
-  total           Int // cents
+  total           Int // cents (includes shipping cost)
+  shippingCost    Int @default(0) // cents — Royal Mail postage cost
 
   // Shipping & tracking info
-  carrier         String? // e.g. "Royal Mail", "DPD", "Courier"
+  shippingService String? // e.g. "Royal Mail 24®", "International Signed" — selected by customer at checkout
+  carrier         String? @default("Royal Mail") // e.g. "Royal Mail", "DPD", "Courier"
   trackingNumber  String? // tracking number from carrier
   estimatedDeliveryDate DateTime? // estimated delivery date