[electrum] improve layout for Trezor firmware tools

Summary: Add a separator and spacings to make it clear the warning text refers to the buttons below. Change the buttons' text color to red.

Test Plan: Display the Trezor Settings, Firmware tab

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17846