[Cashtab] Bump min tx fee to preserve extra token check

Summary:
As a failsafe precaution against token burns, Cashtab assumes any 546-satoshi utxo is an eToken. However, this creates confusion for users because Cashtab will not recognize an incoming minimum transaction of 546 satoshis.

One option is to remove this failsafe precaution. However, I would prefer to keep it in place just in case. Avoiding token burns --- even those that may derive from scenarios not seen in app testing --- I think is more important than recognizing 546-sat transactions in the user's available cash balance.

This diff solves the problem of a user being able to send a tx with cashtab that is not recognized by cashtab (a 5.46 XEC tx). I think having a distinction between eToken dust and XEC dust is useful as a failsafe exclusion option.

T1688

Test Plan: npm start, try to send a 5.46 xec tx, see error msg, receive a 5.5 XEC tx and see balance update

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9865