[Cashtab] Always show send button on send screen

Summary:
T3724

Make sure the Send button is always visible. User can still scroll to review tx.

Test Plan:
npm test

check out the test site to see how it looks / works

https://cashtab-local-dev.netlify.app/

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D17625