Mobile scanner redesign + backend live search
- Scanner page: fullscreen mobile-first layout, Scan/Search/Recent tabs
- Scan tab: auto-start camera, switch camera, vibration/sound feedback
- Valid/invalid fullscreen states, confirm check-in, auto-return to camera
- Search tab: live backend search (300ms debounce), tap card for detail + check-in
- Recent tab: last 20 check-ins, session counter
- Backend: GET /api/tickets/search (live search), GET /api/tickets/stats/checkin
- Admin layout: hide sidebar on scanner page; fix hooks order (no early return before useEffect)
- Back button to dashboard/events (staff → events, others → admin)
- API: searchLive, getCheckinStats, LiveSearchResult; PostgreSQL LOWER cast for UUID
Co-authored-by: Cursor <cursoragent@cursor.com>
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