Hide 'publish to Shopify' button if any variant already has a shopify_variant_id

Products imported before shopify_product_id column was added have null
there, but their variants already carry shopify_variant_id — that's
enough to know the product exists in Shopify. Running shopify:import
once will backfill shopify_product_id for all existing products.

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

app/Filament/User/Resources/ProductResource/Pages/EditProduct.php

@@ -24,7 +24,12 @@ class EditProduct extends EditRecord
                 ->requiresConfirmation()
                 ->modalHeading('Produkt in Shopify anlegen')
                 ->modalDescription('Erstellt das Produkt mit allen Varianten in Shopify und verknüpft die IDs.')
-                ->visible(fn () => ! $this->record->shopify_product_id)
+                ->visible(fn () =>
+                    ! $this->record->shopify_product_id &&
+                    ! ProductSize::where('product_id', $this->record->id)
+                        ->whereNotNull('shopify_variant_id')
+                        ->exists()
+                )
                 ->action(fn () => $this->createProductInShopify()),
             Actions\Action::make('rezept-versionen')
                 ->label('Rezept-Versionen')