Feature: Add third price option for plain/blank products

- Added plainPrice field to Product model in Prisma schema
- Updated database to include new plainPrice column
- Added Plain price input field to product creation form (new/page.tsx)
- Added Plain price input field to product edit form ([id]/edit/page.tsx)
- Updated createProduct server action to handle plainPrice
- Updated updateProduct server action to handle plainPrice

Now supports three price tiers:
- basePrice: for ready-made items
- plainPrice: for plain/blank items (new)
- personalisedPrice: for made-to-order items

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

prisma/schema.prisma

@@ -26,6 +26,7 @@ model Product {
   category    String
   description String
   basePrice   Int // cents — used for ready-made items on /products
+  plainPrice  Int? // cents — used for plain/blank items
   personalisedPrice Int? // cents — used for made-to-order items on /made-to-order; falls back to basePrice if not set
   colors      String // JSON array of hex strings, e.g. ["#17181C","#FBF9F5"]
   colorPhotos String? // JSON object mapping hex -> photo data URL, e.g. {"#17181C":"data:..."}.