refactor: initiate file_size_normalized state as nil

[Samillion]

Changes:

• Initiate file_size_normalized state as nil
• Adjust file_size_normalized in check_path_url() instead of file-loaded event
  • check_path_url() is already called there
  • Should only be adjusted if is_url() and if download button is enabled

The purpose of this refactor:

• Structurally, it makes sense this way
• It prepares possibility to use language locales for download related states in the future