[Cashtab] Use correct type for comparison to dustSats
[Cashtab] Use correct type for comparison to dustSats

Summary:
`currency.dustSats` is a number, 550

`remainder` is a `BigNumber`

Both should be a `BigNumber` to use the `gte` comparison

Test Plan: `npm start`, send etoken txs, confirm you get a change output unless change would be dust

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D11930

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