29 image chunks have an empty bbox {}, and `fm.bbox ?? default` doesn't catch
an empty object, so the crop URL got w=undefined → /api/crop 400 → broken-image
icon. Now validate bbox (w/h > 0); without it, render the image's textual
description instead of requesting an impossible crop.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>