This PR fixes some issues with the CI/CD lint checks (e.g. they are failing on #24). It includes the following changes:

1. Remove the "Backend check" job from lint-check.yml, since this project has no backend
2. Fixes .husky/lint-config to not run linters in the nonexistent backend directory
3. Update husky version from 8 to 9 since it's not working correctly with version 8, and all our other projects this year are using version 9
4. Install eslint-plugin-react-refresh plugin since it's used in the lint config
5. Fix some linter errors (unused imports, import order, lines between imports)