[Cashtab] Patch send button overflow issue

Summary:
T3841

Seeing the send button overflow on Firefox

Add more specific css conditions to prevent this

Test Plan:
npm start and it looks ok, see screenshots

can check this version deployed to cashtab.io

build extension, test with pop-ups, confirm this still looks as before with "Accept" and "Reject" buttons visible even for large txs

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: johnkuney

Differential Revision: https://reviews.bitcoinabc.org/D18379