chore: upgrade to Next.js 16

[wrslatz]

Pull Request

Proposed Changes

Upgrade the project to Next.js 16, since Next.js 14 is EOL and has vulnerabilities. I tested these changes with other maintainers and found no issues, but am labeling this a minor version out of an abundance of caution.

Also, upgrades to ESLint 9, closes #262.

I split several other upgrades and changes out from this PR, to keep the change smaller. Linking them here and in comments.

• Update Octokit to v5 #329
• Upgrade React to 19, additional major dependency updates (Primer, TanStack, tRPC) #412
• Add type checking and linting for tests #411
• Refactor hooks to use React Query instead of useEffect + setState pattern #410

Readiness Checklist

Author/Contributor

• If documentation is needed for this change, has that been included in this pull request
• run npm run lint and fix any linting issues that have been introduced
• run npm run test and run tests
• If publishing new data to the public (scorecards, security scan results, code quality results, live dashboards, etc.), please request review from @jeffrey-luszcz

Reviewer

• Label as either bug, documentation, enhancement, infrastructure, maintenance, or breaking