feat(richtext): half-item overflow scroll affordance in the action bar

When the attachment-editor action bar's items overflow the scroll area, calibrate the inter-item spacing so a half item peeks past the trailing edge (a half-integer count of items fits the viewport at rest), signaling scrollability. Ported from TextProcessingStyleSelectionComponent's half-item slot calibration; rounds the natural fit down to an X.5 so spacing never goes negative.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>