### useCheckinSubmit Hook
- Created `src/hooks/data/useCheckinSubmit.ts` using `postEventCheckinSubmitMutation` from `@/client/@tanstack/react-query.gen`.
- Integrated `sonner` for success and error toasts.
- Followed the pattern from `useJoinEvent.ts`.

### CheckinScannerNav Implementation
- `CheckinScannerNavView` validation logic implemented with regex `^\d{6}$`.
- `CheckinScannerNavContainer` connects the hook to the view.
- Type checking passed with `bun tsc -b`.