Add guest checkout and admin login link

- Add guest checkout flow: customers can now complete orders without creating an account
  - New /checkout page with login or guest options
  - Guest form collects: full name, email, phone, address
  - Orders track guest info via guestName and guestPhone fields
- Update cart message to reflect guest option
- Add "Admin login" link to customer login page for clear navigation
- Update checkout API to accept and store guest information

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

prisma/schema.prisma

@@ -187,6 +187,9 @@ model Order {
   customerId String? // set when the buyer was logged in at checkout — null for guest orders
   customer   Customer? @relation(fields: [customerId], references: [id])
 
+  guestName  String? // full name for guest orders
+  guestPhone String? // contact phone for guest orders
+
   items OrderItem[]
 }