[Cashtab] Improve modal for FIRMA redeems

Summary:
Unlike XECX, users cannot leave $FIRMA redeem offers up pending a 24 hr period. FIRMA price is dynamic, so an offer that is not instantly picked up will soon become mispriced.

If the hot wallet is unable to redeem $FIRMA, disable redeem and show an error.

Test Plan:
npm test

normal $FIRMA redeem

$FIRMA redeem exceeds hot wallet balance

(NB the numbers do not make sense here as I messed with the conditional rendering to get the screenshot, see unit tests for expected behavior here)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18173