[feat] Set up vitest

[thoughtspile]

To make the contributors' job easier, we need to run tests in CI / CD. I suggest:

• adding vitest
• writing a few example tests for the data layer
• adding npm test github action to run on PR

Describe alternatives you've considered

Alternatively, we could start with playwright E2E — the tests would be more stable and allow for easier refactoring. However, it looks harder to get started with.