feat: add avatar stack component

[AbhinRustagi]

Summary

• add a standalone AvatarStack motion primitive for overlapping team/presence avatars
• fan avatars out on hover-capable devices with reduced-motion-safe fallbacks
• register the component with a preview showing compact collaborator presence

Test plan

• bun run typecheck
• bun run lint
• bun run check:registry

Made with Cursor

[starc007]

spreadOnHover doesn't spread the stack — only the single hovered avatar moves. (medium, core interaction)**
avatar-stack.tsx:65 — whileHover is on each <motion.li>, so it fires only for the avatar the pointer is over. That one jumps to its fanned position x = index * expandedStep while its neighbors stay overlapped → gaps and overlaps, not a clean fan-out. The group class on the <ul> is set but never used (group-hover), and the <ul> isn't a motion element, so nothing coordinates a whole-stack spread.
Fix options: drive it from the parent — make the <ul> a motion.ul with whileHover + variants and have each <li> reference variant labels (rest/spread), or apply the fanned x via CSS group-hover on each li. If the intent was only a per-avatar lift, drop x: hoverX and keep y/scale.

Screendrop_2026-06-23-15-05-56.mov

[starc007]

also just a suggestion remove Co-authored-by: Cursor <cursoragent@cursor.com> from the commit message

[AbhinRustagi]

These (avatar, alert, skeleton & spinner) were not supposed to be pushed yet; I didn't realise. My bad! Converting to draft for now

[starc007]

okay okay no worries