Feature: Split products catalog into Plain and Ready-Made

Replaces generic 'showOnProducts' boolean with separate catalog options:
- showAsPlain: for blank/unprinted items (/products/plain)
- showAsReadyMade: for pre-finished items (/products/ready-made)

Changes:
- Updated Product schema to replace showOnProducts with new fields
- Updated product creation and edit forms with new checkboxes
- Updated product listing pages to use showAsReadyMade filter
- Updated ProductDTO type to reflect new catalog flags

Products can now be precisely categorized by type with unique pricing for each.

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

prisma/schema.prisma

@@ -50,7 +50,8 @@ model Product {
   // instead of being shown as a fixed image. Only looks right for black/white/gray
   // source photos — recoloring an already-colored photo produces muddy results.
   showOnPersonalised Boolean @default(true) // list on /made-to-order
-  showOnProducts     Boolean @default(false) // list on /products — same product/photo can show on both
+  showAsPlain        Boolean @default(false) // list on /products/plain — blank/unprinted items
+  showAsReadyMade    Boolean @default(false) // list on /products/ready-made — pre-finished items
   createdAt   DateTime @default(now())
 
   salePrice    Int? // cents — set alongside a date window below to put this product on sale