feat: map prix marché as compare_at_price (barré) on Shopify

When Prix remisé is set: price = remisé, compare_at_price = marché
When no remise: price = marché, compare_at_price = null
Diff now detects compareAtPrice changes to trigger updates

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/shopifySync.ts

@@ -20,6 +20,7 @@ export function getConfig() {
 interface ShopifyVariant {
   id: number
   price: string
+  compare_at_price: string | null
   sku: string
   inventory_quantity: number
 }
@@ -42,6 +43,7 @@ function toSyncProduct(p: ShopifyAPIProduct): ShopifyProductFull {
     title: p.title,
     description: p.body_html ?? '',
     price: p.variants[0]?.price ?? '0.00',
+    compareAtPrice: p.variants[0]?.compare_at_price ?? undefined,
     stock: p.variants[0]?.inventory_quantity ?? 0,
     status: p.status === 'active' ? 'active' : 'draft',
     images: (p.images ?? []).map(i => i.src),
@@ -93,6 +95,7 @@ export async function createShopifyProduct(product: SyncProduct): Promise<string
 
   const variant: Record<string, unknown> = {
     price: product.price,
+    compare_at_price: product.compareAtPrice ?? null,
     sku: product.ref,
     inventory_management: 'shopify',
   }
@@ -179,7 +182,7 @@ export async function updateShopifyProduct(
       title: product.title,
       body_html: product.description,
       status: product.status,
-      variants: [{ id: variantId, price: product.price }],
+      variants: [{ id: variantId, price: product.price, compare_at_price: product.compareAtPrice ?? null }],
     },
   }