Update activity message, email designs in Message bubble

Editor assessment

The subject names the touched surfaces, activity messages and email designs in the message bubble, but 'Update ... designs' says nothing about what visually changed or why, so the diff is mandatory. Feat is a loose type for design revision where style could fit, though the straddle is arguable. No scope, no body, and only a Co-authored-by trailer besides. The trailing pull-request number is conventional. Hygiene is otherwise fine. Understandable as an area label and unhelpful as a description, which keeps it ordinary.