[Cashtab] Patch overlapping text on amount input error Send.js

Summary: Related to T2807. Error text for the Amount input on Send.js is overlapping with the xec to fiar conversion text. Styling has been edited to create enough room for the error text & the conversion text while still keeping the send button in view. Error text font size is now slightly smaller on smaller screens to avoid text wrapping causing content to overlap or be cut off.

Test Plan:
cd web/cashtab && npm start
navigate to send screen
input an incorrect XEC address
input an invalid XEC amount
observe that the error text and the conversion text do not overlap
observe that the content below the ExpandingAddressInputCtn does not jump

in dev tools, try some smaller screens and observe that the text does not overlap

Reviewers: bytesofman, #bitcoin_abc, emack

Reviewed By: bytesofman, #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12640