Add frontend component tests with React Testing Library

[sanmaxdev]

Background

The frontend has no automated tests yet. Adding React Testing Library + Vitest/Jest would let us cover key components and hooks (e.g. frontend/components/ and frontend/lib/hooks/).

Tasks

• Set up a test runner (Vitest or Jest) for the Next.js app
• Add an npm test script
• Write initial tests for a couple of reusable components
• Wire the test step into the CI workflow

Acceptance criteria

• npm test runs locally and in CI
• A clear example exists for others to follow

See CONTRIBUTING.md.