Add operator fee display for optimistic rollups

[tom2drum]

Description and Related Issue(s)

This PR adds support for displaying the operator fee field in transaction details for optimistic rollup chains. The operator fee is an additional fee set by the chain operator to cover extra costs of additional services beyond the standard transaction fees.

Resolves #2999

Proposed Changes

• Added operator_fee field to the Transaction type in types/api/transaction.ts
• Updated transaction details UI to display operator fee for optimistic rollups in ui/tx/details/TxInfo.tsx
• Added operator fee to L2 transaction mock data in mocks/txs/tx.ts
• Updated visual regression test screenshot for L2 transactions

Breaking or Incompatible Changes

None. This is a backward-compatible addition that only displays the operator fee when the field is present in the API response.

Additional Information

The operator fee is only displayed when:

1. The rollup feature is enabled
2. The rollup type is 'optimistic'
3. The transaction data includes an operator_fee field

The fee is displayed with proper currency formatting and includes an exchange rate conversion when available.

Checklist for PR author

• I have tested these changes locally.
• I added tests to cover any new functionality, following this guide
• Whenever I fix a bug, I include a regression test to ensure that the bug does not reappear silently.
• If I have added, changed, renamed, or removed an environment variable
  • I updated the list of environment variables in the documentation
  • I made the necessary changes to the validator script according to the guide
  • I added "ENVs" label to this pull request