Fix ESLint warnings and remove from ignore list (#48291)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/48291

Changelog: [internal]

`react-native-fantom` was added to `.eslintignore` when we upgraded to React 19 because we wanted to reduce the amount of warnings to ease the migration.

This re-enables ESLint for that directory and removes the warnings.

Reviewed By: cipolleschi

Differential Revision: D67283104

fbshipit-source-id: 4ec2363ceaff3cd7bd6e5d70e9588c3dd22d7d85