Update example app to use deferredPurchaseStream

[NickSxti]

Summary

• Subscribe the example app to Qonversion.deferredPurchaseStream during SDK initialization, mirroring the existing No-Codes stream subscription pattern in main.dart.
• On a successful deferred purchase, refresh entitlements in AppState so other screens reflect the new state.
• Track each deferred purchase event in AppState (with a clearable history) so future screens can display them if needed.

Notes

• The example app previously had no setEntitlementsUpdateListener usage to migrate from. This PR just adds the new API hookup.
• The stream emits QPurchaseResult (not QDeferredTransaction) - matching lib/src/qonversion.dart and lib/src/dto/purchase_result.dart.

Test plan

• Build and run the example app on iOS / Android
• Trigger a deferred purchase (e.g. Ask to Buy / SCA in sandbox)
• Confirm the debug log prints Deferred purchase ... and entitlements update on success

[NickSxti]

Closing - no existing entitlements update listener usage in the example app to migrate from. The change would have added a new feature demo rather than performing a migration, which is out of scope of the original request.