feat: collection picker in product creation flow

- API /api/creation/collections liste toutes les collections Shopify
- Détection automatique depuis la colonne ID FAMILLE du Sheets
- Sélecteur avec recherche si absente ou pour modifier
- Collection affichée dans l'écran de confirmation
- collectionOverride passé à l'exécution comme fallback

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

src/app/api/creation/execute/route.ts

@@ -20,9 +20,10 @@ export async function POST(req: NextRequest) {
   }
 
   const userId = (session.user as { id: string }).id
-  const body = await req.json().catch(() => ({})) as { resolutions?: MetafieldResolution[]; tab?: string }
+  const body = await req.json().catch(() => ({})) as { resolutions?: MetafieldResolution[]; tab?: string; collectionOverride?: string }
   const resolutions: MetafieldResolution[] = body.resolutions ?? []
   const tabFilter: string | undefined = body.tab ?? undefined
+  const collectionOverride: string | undefined = body.collectionOverride ?? undefined
   const resolutionMap = new Map<string, MetafieldResolution>(resolutions.map(r => [r.columnHeader, r]))
 
   const stream = new ReadableStream({
@@ -83,7 +84,8 @@ export async function POST(req: NextRequest) {
               weightKg: product.weightKg,
               status: 'active',
             })
-            if (product.collectionId) await addProductToCollection(shopifyId, product.collectionId)
+            const effectiveCollectionId = product.collectionId || collectionOverride || ''
+            if (effectiveCollectionId) await addProductToCollection(shopifyId, effectiveCollectionId)
             if (product.subCollectionId) await addProductToCollection(shopifyId, product.subCollectionId)
             send({ type: 'progress', current: i + 1, total, tab: product.tab, title: product.title, step: 'Métachamps…' })
             for (const [colHeader, value] of Object.entries(product.specificFields)) {