[cashtab] Fix header copy address button hover styles

Summary:
Reported that the copy address button in the header, on mobile, would hang on the hover state background color
Moving the hover styles to a more specific selector

Test Plan: preview the site and check out the button on mobile

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18104