F18: Friends and groups — shared packs, versus mode (feasibility)

## What to build

Investigate and prototype social features: shared packs where multiple users contribute completions, and a versus mode for friendly competition. This is a feasibility slice — produce a working prototype and a recommendation on full implementation scope.

## Acceptance criteria

- [ ] Feasibility assessed: data model for shared packs and group membership
- [ ] Prototype: at least two accounts can share a pack and see each other's completions
- [ ] Versus mode concept prototyped: leaderboard or head-to-head XP comparison
- [ ] Privacy and permission model sketched (who can invite, who can see what)
- [ ] Findings documented: effort estimate, edge cases, recommendation to proceed or not

## Blocked by

- #15 (F16 — multi-device accounts are the foundation for any multi-user features)