[Cashtab] Better extension popup size

Summary:
When the extension is opened by a webapp, the Send button should be fully visible.

Bump dimensions to get this.

Test Plan: npm run extension, confirm size looks good (or review screenshots)

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15156