Setup testing framework for explorer app

The explorer app currently lacks a structured testing setup. To improve reliability, prevent regressions, and make future refactors safer, we should introduce a proper testing framework aligned with our monorepo setup.

Start by inspecting the monorepo's existing testing setup and locating the explorer app's entry point and configuration. Determine which framework and runner fit the current structure; done means the app has a repeatable testing setup and at least one test can be run through the repository's standard workflow.

Written by the indexing model from the issue text.