Set up Vitest and React Testing Library

[manerlan]

Sets up the testing environment for the project using Vitest and React Testing Library.

Changes:

• Added Vitest as the test runner.
• Added jsdom for a browser-like test environment.
• Added React Testing Library and jest-dom.
• Added user-event for future interaction tests.
• Added Vitest configuration to vite.config.ts.
• Added setupTests.ts for shared test setup.
• Added an initial component test for FavoritesSidebar to verify that the testing setup works.
• Ran Prettier on the related files.

For Testing:

• npm test -- --run
• npm run lint
• npm run build

Closing this issue and making a new issue for the required snapshot tests, further component tests for props/state/user interaction, and API mocking.

[oscarba]

Vitest implementert, funker.