feat(leaderboard): per-faction data freshness (#212)

[dmccoystephenson]

Summary

Adds a "Updated N days ago" line to each faction row (from faction.updatedAt), so visitors can judge whether a member count is current. Backed by a new pure helper utils/relativeTime.relativeTimeFrom(iso, now) (now passed in for testability).

Test plan

• npm test 74 pass (5 new in relativeTime.test.ts: just-now, minutes sing/plural, hours, days, unparseable).
• npm run lint clean, npm run build succeeds. Verified the Server cell renders once (no markup duplication).

Closes #212

[dmccoystephenson]

Self-review rubric (CI green):

• Scope: PASS — new relativeTime util + test, leaderboard cell, CHANGELOG; all for Leaderboard: no indication of how fresh the faction data is #212.
• Tests-new: PASS — relativeTimeFrom has 5 tests covering each unit + plural + invalid input.
• Sibling structure: PASS — util matches utils/* (pure fn, now-injected for testability); test mirrors sortPlugins/likedItems.
• Self-caught defect: an over-broad first edit duplicated the Server ; removed it and verified exactly one label={faction.serverId} remains + clean build.
• Issue resolution: PASS — per-row freshness shown from updatedAt.
• Docs: PASS — CHANGELOG. CI: PASS. no-any: PASS. mui-only: PASS. env-vars-documented: N/A.