Add basic GitHub Actions CI pipeline

[Copilot]

Summary

• add a CI workflow that runs on pull requests and pushes to main using Node 22
• run npm ci, npm run lint, npm run build, and npm test in GitHub Actions
• add Vitest and a few small mocked-data tests for Codeforces service helpers

Testing

• npm test
• npm run lint
• npm run build

[Copilot]

Pull request overview

Adds a basic GitHub Actions CI workflow for the project and introduces a minimal Vitest test suite to validate key Codeforces service helper behavior, ensuring lint/build/test run automatically on PRs and main pushes.

Changes:

• Added a GitHub Actions workflow that runs npm ci, npm run lint, npm run build, and npm test on PRs and pushes to main (Node 22).
• Introduced Vitest (npm test) and added helper-focused unit tests for Codeforces daily-problem logic and failure handling.
• Exported getProblemKey and pickDailyProblem to make them directly testable.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
.github/workflows/ci.yml	Adds CI workflow to run install/lint/build/test on PRs and main pushes using Node 22.
package.json	Adds test script (vitest run) and includes vitest in devDependencies.
package-lock.json	Locks Vitest and its transitive dependencies for reproducible installs in CI.
src/services/codeforcesService.js	Exports helper functions needed for unit tests (getProblemKey, pickDailyProblem).
src/services/codeforcesService.test.js	Adds Vitest unit tests for helper behavior and problemset failure fallback behavior.

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