Fix measurement display and ruler rendering

- Add printAreaWidthMm and printAreaHeightMm to ProductDTO type
- Include these fields in toProductDTO conversion function
- Refactor ruler calculations to use simple for loops
- Rulers now display correctly showing 2cm increments on both axes
- Position & Size panel now shows accurate cm measurements

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

src/lib/product.ts

@@ -19,6 +19,8 @@ export function toProductDTO(p: PrismaProduct, activePromotions: ActivePromotion
     mockup: p.mockup,
     printArea: JSON.parse(p.printArea),
     printAreaBack: p.printAreaBack ? JSON.parse(p.printAreaBack) : null,
+    printAreaWidthMm: p.printAreaWidthMm,
+    printAreaHeightMm: p.printAreaHeightMm,
     imageUrl: p.imageUrl,
     imageUrlBack: p.imageUrlBack,
     photoRecolorable: p.photoRecolorable,

src/lib/types.ts

@@ -43,6 +43,8 @@ export type ProductDTO = {
   mockup: string;
   printArea: PrintArea;
   printAreaBack: PrintArea | null;
+  printAreaWidthMm: number;
+  printAreaHeightMm: number;
   imageUrl: string | null;
   imageUrlBack: string | null;
   photoRecolorable: boolean;