feat(ui): annotation card header slot and @ mentions on the card's edit box; ui 0.45.0 (#1578)

AnnotationPanel.renderCardHeader is the twin of renderCardFooter: a host node rendered in each plan-annotation card's header row, in a wrapper that swallows click and keydown, also under readOnly, absent means no element. AnnotationPanel.mentionSource gives the card's edit box the same @ picker as the composer; a save sends onEdit({ text, mentions }) only when someone was picked during that edit and the token survived, and exactly onEdit({ text }) otherwise, so an untouched edit never wipes existing tags. The edit box is extracted into AnnotationEditComposer and the mention menu into a shared MentionAutocomplete unit used by both composer sites and the card. With neither prop the panel and the composer are byte-identical and behavior-identical to before; code-review cards are untouched.