Small improvement for mini apps: improved transaction previews

Working on Farcaster and Warpcast. Longer thoughts at https://dwr.email

Where is the best place to report bugs 👀? Thanks!

Have noticed that some swaps work on warpcast web vs not working on warpcast mobile. 

This bug isn't exactly mini-app transaction preview related. However a swapPreview action in the sdk would be useful 👀