[Cashtab] Correct horizontal scrollbar in balance header on desktop and extension

Summary:
We get the system default scrollbar at screen widths < 1100px for non-mobile devices. The styles are optimized for touchscreen devices, but desktop windows at smaller widths (like the extension) do not support this, and just show an ugly system default scrollbar.

Add a customized scrollbar for this case. Keep the mobile device stuff unchanged.

Test Plan:
Check with mobile device and desktop at cashtab.io, check extension, npm test, screenshots

extension before

extension after

mobile before and after unchanged

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18217