Fix problems in the demo app

[danielsaidi]

The demo needs some adjustments after iOS 18 and the Swift 6 changes.

First of all, the SDKs Swift 6 transition causes some warnings in the demo. But it seems like many of the native document APIs are not playing that well with Swift concurrency.

Furthermore, the iOS demo looks strange when running on an iPhone with iOS 18. When running the demo, the text editor now expands outside of the edges of the screen.

Any help to address these problems would be greatly appreciated.