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/migrations/20260719054459_add_product_weight/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "Product" ADD COLUMN     "weightGrams" INTEGER;

prisma/migrations/20260719054540_add_shipping_fields/migration.sql

@@ -0,0 +1,5 @@
+-- AlterTable
+ALTER TABLE "Order" ADD COLUMN     "shippingCost" INTEGER NOT NULL DEFAULT 0,
+ADD COLUMN     "shippingCountry" TEXT,
+ADD COLUMN     "shippingService" TEXT,
+ALTER COLUMN "carrier" SET DEFAULT 'Royal Mail';