Add: Shipping selector component and checkout integration

Added ShippingSelector component that:
- Displays country selection dropdown
- Fetches available Royal Mail shipping quotes based on cart weight
- Shows delivery estimates and international delivery warnings
- Lets customers select preferred shipping service
- Auto-selects cheapest option by default

Integrated into checkout:
- Shows shipping selector in both logged-in and guest flows
- Displays shipping cost breakdown in order summary
- Passes shipping country and service to checkout API
- Validates shipping selection before processing payment

.claude/settings.local.json

@@ -87,7 +87,8 @@
       "Bash(git commit -m 'Refactor: Move start design server action to separate file *)",
       "Bash(git commit -m 'Add: Prominent '\\\\''Shop plain items'\\\\'' section to homepage *)",
       "Bash(git commit -m 'Feature: Royal Mail shipping integration with weight-based postage *)",
-      "Bash(git commit -m 'Add: Weight field to product admin pages *)"
+      "Bash(git commit -m 'Add: Weight field to product admin pages *)",
+      "Bash(git commit -m 'Add: Shipping selector component and checkout integration *)"
     ]
   }
 }