Website Sale Attribute Filter Multiselect

This module extends the Website Sale e-commerce features. It adds a new option called 'Multiple Select' to the display type of product attributes (product.attribute), alongside the existing radio, select, color and image options.

When an attribute is set to this new type, customers can tick several values at the same time instead of choosing only one:

• On the product detail page, the attribute is shown as a multi-choice dropdown, with price extras displayed next to each value when they exist.
• In the shop's attribute filter (both the normal filter area and the off-canvas mobile filter), the attribute appears as a multi-choice list with an 'Apply' button to confirm the selection.

The module also changes how the variant selector behaves: the standard single-value change logic is skipped for these multi-select fields, so ticking several values does not trigger the usual one-value-only behaviour. The multi-select widget includes a search filter and a clear button, and its labels follow the visitor's language.

If the 'Multiple Select' option is later removed from an attribute, existing attributes of that type are automatically switched back to 'Radio' to avoid breaking the website. The module touches the product attribute model and the website sale product, filter and off-canvas templates.