feat: #3628610 Allow longer image captions

Replaces custom caption and description textfields with textareas in both image formatters, removing the default 128-character limit. Stored settings remain strings.

Local browser checks on Drupal 11.4.8 confirm both inputs retain 740 characters in both formatters.

Issue: https://www.drupal.org/project/glightbox/issues/3628610 Equivalent video proposal: https://www.drupal.org/project/glightbox_media_video/issues/3628605

AI-Generated: Yes (Codex prepared the formatter changes.)