Add getLeaveStatus utility and update project status

- Introduce `getLeaveStatus` function in `packages/core/src/status-utils.ts` to determine leave-by status based on
  journey data
- Mark Phase 1 mobile app tasks as complete in `CHECKLIST.md`
- Add mobile workspace configurations and npm scripts

apps/mobile/App.tsx

@@ -0,0 +1,5 @@
+import AppNavigator from './src/navigation/AppNavigator';
+
+export default function App() {
+  return <AppNavigator />;
+}