fix: enforce stock limits on basket quantities

No stock validation existed in the ordering flow, allowing customers
to order more items than available.

Cap quantity to ``stock_current`` in ``Baskets::getBasketData()`` when
adding to cart. Add ``min=1`` and ``max=stock`` attributes on the
basket quantity input, with JS clamping in the change handler.
Verify stock again in ``Shop\OrderController::store()`` before saving
the order as a race-condition safeguard.

app/Repositories/Shop/Baskets.php

@@ -116,6 +116,7 @@ class Baskets
                 'variation' => $offer->variation->name,
                 'image' => Articles::getPreviewSrc(ArticleImages::getFullImageByArticle($offer->article)),
                 'latin' => $offer->article->product->specie->latin ?? false,
+                'stock' => $offer->stock_current !== null ? (int) $offer->stock_current : null,
             ];
         }
 
@@ -144,6 +145,13 @@ class Baskets
     {
         $offer = Offers::get($id, ['article', 'variation']);
 
+        if ($offer && $offer->stock_current !== null) {
+            $quantity = min($quantity, max(0, (int) $offer->stock_current));
+            if ($quantity <= 0) {
+                return false;
+            }
+        }
+
         return [
             'id' => $id,
             'name' => self::getArticleName($offer),