CSV import character encoding is now automatically verified

 If a file is not in UTF-8 format and contains special characters or diacritics, the import will be stopped and a clear error message will be shown to the user.

This prevents common issues where special characters can break database entries or result in incorrect product data. Users are instructed to re-save the file using UTF-8 encoding and try the import again.

This improvement ensures data integrity and makes error tracing significantly easier during import operations.