chore: address eslint errors

[cheahjs]

Pull Request Checklist

• [x] Target branch: Pull requests should target the dev branch.
• [ ] Description: Briefly describe the changes in this pull request.
• [ ] Changelog: Ensure a changelog entry following the format of Keep a Changelog is added at the bottom of the PR description.
• [ ] Documentation: Have you updated relevant documentation Open WebUI Docs, or other documentation sources?
• [ ] Dependencies: Are there any new dependencies? Have you updated the dependency versions in the documentation?
• [ ] Testing: Have you written and run sufficient tests for the changes?
• [ ] Code Review: Have you self-reviewed your code and addressed any coding standard issues?

---

Description

1. Address all eslint errors that are flagged when running npm run lint:frontend
2. Add types for i18next usage across the codebase
3. Add eslint-plugin-unused-imports to remove unused imports