[Cashtab] Use 64-bit ints for agora offers

Summary: There's not really a reason to use 32 anymore. Will move the ecash-agora default to 64, but that will require some test updates. For now, just update the most common tool used for creating offers.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18938