🧪 Testing - Add Unit Testing

Note: Only action once all pages have been migrated

As per my conversation with @wicksipedia, it would be good to add a testing framework (i.e. Jest) to the SSW Website.
Storybook should also be considered alongside Jest to also allow visual testing and a visual database of the components in the project.

### Tasks
- [ ] Investigate whether to add Jest and/or Playwright and/or other testing framework to the project
- [ ] Add the chosen framework 
- [ ] Add a way to measure code coverage using these unit testing frameworks

Links:
https://nextjs.org/docs/pages/building-your-application/optimizing/testing#jest-and-react-testing-library
https://storybook.js.org/docs/writing-tests/test-runner

Wait until all pages have been migrated, then review the linked Next.js and Storybook testing guidance and compare Jest, Playwright, and other candidate frameworks. Decide which framework or combination fits the project, add it, and provide a code-coverage measurement method; completion should satisfy all three listed tasks.