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/Articles.php

@@ -43,6 +43,7 @@ class Articles
                 'id' => $offer->id,
                 'name' => $offer->variation->name,
                 'prices' => $offer->tariff->price_lists->first()->price_list_values->toArray(),
+                'stock' => $offer->stock_current !== null ? (int) $offer->stock_current : null,
             ];
         }