feat:Add a "Recently Completed" hunts section to the Arcade (#359)

[Awointa]

What changed

• HuntCards now shows a live registered player count and a 🔥 Trending badge
  when a hunt exceeds 50 players.
• The arcade page shows a horizontally scrollable "Recently Completed" strip
  (up to 5 hunts) with winner addresses and a "View results" link per card.
• Player counts are cached in-memory (60 s TTL) and batch-fetched on each
  arcade page load to avoid N sequential calls.

New files

┌────────────────────────┬────────────────────────────────────────────────┐
│ File │ Purpose │
├────────────────────────┼────────────────────────────────────────────────┤
│ hooks/usePlayerCo │ Single-hunt count with TTL cache and no-throw │
│ unt.ts │ guarantee │
├────────────────────────┼────────────────────────────────────────────────┤
│ hooks/usePlayerCo │ Bulk fetch via Promise.allSettled; exposes │
│ unts.ts │ refetch() │
├────────────────────────┼────────────────────────────────────────────────┤
│ hooks/useRecently │ Derives completed hunts from existing query; │
│ Completed.ts │ slices to MAX_RECENTLY_COMPLETED = 5 │
├────────────────────────┼────────────────────────────────────────────────┤
│ components/Comple │ Separate card for completed hunts (distinct │
│ tedHuntCard.tsx │ data shape from HuntCards) │
├────────────────────────┼────────────────────────────────────────────────┤
│ components/Recent │ Returns null when empty; horizontal scroll, │
│ lyCompletedSection.tsx │ keyboard-navigable │
├────────────────────────┼────────────────────────────────────────────────┤
│ hooks/usePlayerCo │ 10 tests: storage scan, TTL cache, trending │
│ unt.test.ts │ threshold, stale re-fetch │
├────────────────────────┼────────────────────────────────────────────────┤
│ hooks/usePlayerCo │ 7 tests: bulk fetch, cache hit, refetch, │
│ unts.test.ts │ partial failure isolation │
├────────────────────────┼────────────────────────────────────────────────┤
│ components/test │ 20 tests: count display, badge, a11y, card │
│ s/HuntCards.test.tsx │ rendering, answer submission │
└────────────────────────┴────────────────────────────────────────────────┘

Modified files

• lib/types.ts — added TRENDING_PLAYER_THRESHOLD, PLAYER_COUNT_CACHE_TTL_MS,
  PlayerCountResult
• components/HuntCards.tsx — optional playerCount/isTrending props; falls back
  to usePlayerCount in the play flow
• app/page.tsx — calls usePlayerCounts + useRecentlyCompleted; renders
  RecentlyCompletedSection

Design decisions

• Player counts are read from localStorage keys
  (hunt_registered_{id}_{address}), consistent with how registerPlayer and
  getPlayerProgress already work — no new RPC call.
• RecentlyCompletedSection returns null (not an empty container) when there
  are no completed hunts.
• Raw winner addresses never appear as visible text — always truncateAddress;
  full address on title + aria-label.
• CompletedHuntCard is a separate component; sharing HuntCards would require
  nullable props that make the type system unhelpful.

Testing

All 37 new tests pass. Pre-existing failures in huntStore.test.ts and
GlobalActivityFeed.test.tsx are unrelated to this PR (pre-existing
localStorage.clear and logger issues in the test environment).

closes #359