Fixed notification +N count badge alignment in ActivityPub (#28290)

ref
https://linear.app/ghost/issue/DES-1400/notification-more-count-15-etc-alignment-is-off-in-activitypub

- the avatar stack row stretched to full width, so the absolutely
positioned +N badge anchored to the far right edge instead of the last
avatar
- added w-fit so the row shrinks to its content, letting the existing
right offset land the badge on the last avatar