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)

這是一項針對社交功能的設計與可行性研究。首先檢查 pack 和 completion 的資料模型，可能位於後端資料庫結構中。調查關於多裝置帳戶基礎的 issue #15。為共用 pack 成員關係和簡單的對戰排行榜製作原型。記錄有關工作量和邊界情況的調查結果。

由索引模型根據 Issue 內容生成。