Extended the method existence checks throughout src/QUI/ERP/Order/Basket/ files. Improved the
robustness of the code by ensuring that methods such as 'getUuid', 'getProductSetParentUuid',
'getQuantity', 'toArticle', and 'getAttributesForUniqueField' exist before calling them, thus
preventing potential undefined method errors. Removed unnecessary comments used for type hinting in
favor of functional checks.

Related: #172

Improve readability and interpretability of code by explicitly stating null in union types. This
change has been applied across multiple files and functions.

PHP8.4 compatibility

Added a conditional check in src/QUI/ERP/Order/Basket/Basket.php to convert basketId to actual id
from user's session if it is a boolean value. This fixes potential issues that may arise with
non-integer basketIds.

Also, introduced a safeguard check before saving to ensure there is a valid User. If not, the save
function is exited early to prevent potential errors or undesired actions.

related: quiqqer/order-guestorder#8

feat: merge shopping cart and order if products are different. if products were placed in the shopping cart in the status not logged in

feat: user currency -> the user can now have his own currency, so he can use the exchange rate to display the price in his currency.