Install and configure Vitest as a test runner

[andreaskienle]

For component tests, we use Cypress. For (non-component) function or unit tests, it would be beneficial to install a dedicated test runner such as Vitest.

During the refinement, it has been decided to go for Vitest.

To implement:

• Install and configure the test runner
• Rename Cypress test files (.test.tsx → .cy.tsx)
• stretch: Select a simple function and create a sample unit test
• stretch: Run tests in GHA (could also be a follow-up task)

There may be some challenges due to potential conflicts between Cypress and the unit test runner, because of global variables like expect or assert.