[Cashtab] standardize balance header components

Summary: Pull shared components from a single file (Atoms.js) instead of duplicating them. Minor formatting tweaks to ensure the same format across screens.

Test Plan: npm start and visually inspect Wallet, Send, and SendToken screens to confirm balance header looks ok.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9480