cmdpal: fix an AOT-only crash loading the extension page (#48065)

Okay I'll admit this is a clanker fix. 

There's currently a crash when you navigate to an individual extension
page, that only repro's in Release/AOT. And because of
release&trimming&AOT reasons, that's very very difficult to diagnose.
But the clanker added a shitload of logging, and was able to figure out
that:

We were binding to this `Screenshots` collection. Problem is that we
can't safely bind to an `IReadOnlyList` in trimmed scenarios. C#/AOT is
a wonderful world of horrors.

Fixing this is as simple as swapping it for a `ObservableCollection`.