[Cashtab] Improve minified nav header to prevent content jumping

Summary:
T3564

Use position: sticky to stop content jumping when the minified header is activated.

Test Plan:
npm test

visit https://cashtab-local-dev.netlify.app/ and scroll, no content jumping

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D16122