Update CustomizeGasScreen Send Amount/New Total/Fiat Conversion for Tokens.

Original discussion from #8150, where the Send Amount on the Customize Gas screen while editing the gas is hard coded to be ETH. OP would see this is be dynamic with the token being sent. This can be easily changed, but would need feedback on what to show for long decimal tokens, and the New Total and the Fiat Currency values.

Possible option is to convert the Token Send Amount to ETH and add it accordingly to those values.

Start at the CustomizeGasScreen entry point and trace how Send Amount, New Total, and Fiat Currency are calculated while editing gas. Resolve with maintainers how token amounts and long decimals should display, then define the expected ETH conversion and confirm all three values update consistently.

Written by the indexing model from the issue text.