Set lint to fail on warnings (front end)

**Describe the task**
I don't like warnings. Make them go away.

**Acceptance Criteria**
- [ ] Warnings in lint should cause fail in pipeline
- [ ] Fix warnings

**Additional context**
- New version of eslint is more strict.

Start by locating the front-end ESLint configuration and the pipeline lint step; the issue names no files or failing warnings. Enable lint warnings to fail the pipeline, fix the warnings introduced by the stricter ESLint version, and verify both acceptance criteria.

Written by the indexing model from the issue text.