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/index.ts

@@ -0,0 +1,8 @@
+import { registerRootComponent } from 'expo';
+
+import App from './App';
+
+// registerRootComponent calls AppRegistry.registerComponent('main', () => App);
+// It also ensures that whether you load the app in Expo Go or in a native build,
+// the environment is set up appropriately
+registerRootComponent(App);