Fix/checkboxlist multiple values

[skrezh]

When a checkboxlist property has multiple values selected (e.g. ["Drinks", "Sauces"]), the filter should allow selecting a single option and show all products where that option is part of the array.

Problem

The checkboxlist property type does not work correctly when multiple values are selected. When a product has several values (e.g., ["Drinks", "Sauces"]), the following issues occur:

1. PropertyValue::getValueAttribute() returns raw JSON string instead of an array
2. PropertyValue::getDisplayValueAttribute() displays raw JSON instead of a comma-separated list
3. PropertyValue::beforeSave() generates an incorrect index_value (slug of the whole JSON string)
4. UniquePropertyValuesInCategoriesQuery groups by the entire JSON array, so multi-value entries don't appear as separate filter options
5. ProductEntry::mapProps() / VariantEntry::mapProps() don't split checkboxlist values into individual index entries, breaking JSON_CONTAINS filtering
6. Property::getValuesForCategory() throws "Cannot access offset of type array" when sorting values
7. PropertyFields::checkboxlist() passes array to json_decode() causing a TypeError in backend

Changes

• models/PropertyValue.php: Handle checkboxlist type in getValueAttribute() (return array), getDisplayValueAttribute() (comma-separated string), and beforeSave() (dot-separated slugs as index_value)
• classes/queries/UniquePropertyValuesInCategoriesQuery.php: Add get() method that expands JSON arrays into individual filter entries
• classes/index/ProductEntry.php & VariantEntry.php: Split dot-separated index_value into individual array elements in mapProps()
• models/Property.php: Fix sorting when value is an array
• formwidgets/PropertyFields.php: Fix json_decode() receiving array instead of string