[Cashtab] Autosize the wallet select based on text width

Summary:
Currently we get an approximation that does not account for differing widths of different characters.

Get an exact value. Also, use the app font for this select menu.

Test Plan:
npm test, note we can't jest unit test this html-specific method, can view impact with npm start

View on test site at https://cashtab-local-dev.netlify.app/

change some wallets

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15774