Add GitHub Actions CI for tests and linting

[TimBest]

Summary

• Add a .github/workflows/ci.yml workflow that runs on push to main and on pull requests, with two parallel jobs: lint and test
• Add a lint script to package.json (eslint src --ext .js,.jsx,.ts,.tsx), reusing the existing eslintConfig (extends: react-app) — no new dependencies required since eslint is already pulled in transitively by react-scripts
• Both jobs use Node 20 with NODE_OPTIONS=--openssl-legacy-provider (matches the working Netlify build config, required for react-scripts 3.4.4/webpack 4 on modern Node/OpenSSL) and cache yarn dependencies
• test job runs Jest non-interactively via yarn test --watchAll=false with CI=true

Test plan

• yarn install --frozen-lockfile succeeds locally
• yarn lint passes with no errors locally
• CI=true yarn test --watchAll=false passes locally (11 test suites, 53 tests)
• GitHub Actions lint and test jobs pass on this PR

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