[Bug]: Save custom gas fee settings option missing

## Describe the bug
The option to save custom gas fee settings for future transactions is missing from the Edit gas fee modal. This feature existed in older versions of MetaMask but is no longer available in recent versions (v13.x).

This issue is particularly problematic for custom networks (e.g., Shape Network with chainId 360) where users frequently need to adjust gas settings.

## Expected behavior
Users should be able to save their custom gas fee preferences via a checkbox (e.g., `Save for future transactions`) so values persist across transactions.

## Steps to reproduce
1. Open MetaMask v13.18.0
2. Connect to a custom network (e.g., Shape Network, chainId 360)
3. Start any transaction
4. Click `Edit` on gas fees
5. Modify gas fee values
6. Look for a `Save for future transactions` checkbox - it does not exist
7. Confirm the transaction
8. Start new transaction - gas fees reset to defaults

## Version
13.18.0

## Where was this bug found?
Live version (from official store)

## Browser
Chrome

## Operating system
Windows

## Additional context
This feature was available in older MetaMask versions but was removed or broken in recent updates. The issue is especially impactful for custom networks like Shape (chainId 360) where users need to set gas fees manually and would benefit from saving those preferences. Please restore the ability to save custom gas fee settings for future transactions.

Start by reproducing the missing option in the Edit gas fee modal on MetaMask v13.18.0 with a custom network such as Shape Network (chainId 360), then trace the modal and transaction gas-settings flow. Done means users can save custom gas fee values for future transactions and those values persist when starting a new transaction.

Written by the indexing model from the issue text.