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>

src/app/products/page.tsx

@@ -47,7 +47,7 @@ export default async function ReadyMadeProductsPage({ searchParams }: { searchPa
   }
   const palette = Array.from(paletteSet);
 
-  const where: Prisma.ProductWhereInput = { showOnProducts: true };
+  const where: Prisma.ProductWhereInput = { showAsReadyMade: true };
   if (selectedCategories.length) {
     where.category = { in: selectedCategories };
   }