fix: use actual input output difference

[0xApotheosis]

Description

Updates the value shown in the "buy amount fiat difference" part of the trade input to actually show the percentage difference in the fiat amount received vs the fiat amount sent (excluding gas).

Previously, this was showing the price impact, which did not match the copy.

Issue (if applicable)

Closes #7790

Risk

High Risk PRs Require 2 approvals

Small

What protocols, transaction types, wallets or contract interactions might be affected by this PR?

None

Testing

• Ensure the input/output percentage looks correct for a range of trades with and without fees.
• Ensure the correct color-coding is used: white if less than 1%, yellow if less than 3%, red if greater than 3%. Green if positive!

Engineering

☝️

Operations

• 🏁 My feature is behind a flag and doesn't require operations testing (yet)

☝️

Screenshots (if applicable)

[NeOMakinG]

Very small comment, pending runtime test

Looks good overall, the logic seems straightforward

[NeOMakinG]

All good! ✅