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I like the idea of just keeping the default messages app/widget with the library and deferring to custom apps if installed.

Yes, I think this makes a lot of sense. Having one extra file (for the app) is not a big deal even if it's not used, and it would still be good for managing messages.

Maybe later we can split it out, but right now I think it makes a lot of sense to keep it.

Maybe we could even do something like this:

Yes - that looks great to me. That seems like a really neat pattern that we could probably use in other places too.