feat(07-03): create mobile engagement page (plan 01 endpoints + plan 02 components)
- New app/mobile/engagement/page.tsx ('use client', 378 lines)
- Period chips (D30 default), 3 independent fetches on mount/period change
- Sort chips (hours default), refetch users only on sort change
- Client-side search filter (useMemo, 300ms debounce via EngagementSearchInput)
- IntersectionObserver infinite scroll (rootMargin 200px) + Load more fallback
- 4 summary card skeletons + sparkline skeleton + 5 row skeletons on initial load
- Empty state (activeUsers === 0 + users.length === 0), not-configured banner, no-matches inline
- toast.error per failing fetch; Load more flips to Retry on error
- BottomNav and MoreDrawer unchanged (ENG-09 / D-01 / D-02)
lorentz
5daf7f31e5