Fixed: the embed widget, toolbar-button and balloon-form CSS did not load on the content editing surface — it was attached via admin_library, which Drupal's CKEditor 5 only loads on the text-format configuration page. The brand-icon SVG (no intrinsic size) then expanded to the full editor width. The CSS now loads via the editing-surface library.
Added Drupal 12 compatibility (core_version_requirement and composer constraint).