Add product dimensions and downloadable design specification sheet

- Add printAreaWidthMm and printAreaHeightMm fields to Product model
- Create API endpoint to generate design specs with CM measurements
- Add download button to order detail page
- Specs include design elements with positions/sizes in centimeters

src/app/admin/orders/[id]/page.tsx

@@ -23,6 +23,12 @@ export default async function OrderDetailPage({ params }: { params: { id: string
         <h1 className="font-display text-3xl">Order</h1>
         <div className="flex items-center gap-3">
           {order.archived && <span className="tag-label">Archived</span>}
+          <a
+            href={`/api/orders/${order.id}/design-spec`}
+            className="tag-label border border-line px-3 py-1.5 hover:border-clay hover:text-clay"
+          >
+            ↓ Download specs
+          </a>
           <form action={order.archived ? unarchiveOrder : archiveOrder}>
             <input type="hidden" name="id" value={order.id} />
             <button type="submit" className="tag-label border border-line px-3 py-1.5 hover:border-clay hover:text-clay">