Idea for new Export PSBT button to match functionality of Import PSBT button

It would be nice to have an "Export Current PSBT" button on the signing page, that includes any signatures that have already been gathered in the downloaded file (and one wouldn't need to unnecessarily enter the Coldcard flow).

That way, the file could be shared with a collaborative multisig parter, who uses the "Import PSBT" button to stage the transaction with all current progress, apply additional signatures, and then broadcast or continue to pass it along.