Setup tests

[dotnize]

I plan to implement this in tanstarter-monorepo, but should we implement this here too?

goals:

• minimal
• should be optional and can be ignored, or even removed easily (for MVPs and quick prototypes that don't need tests)
• both focused unit tests and integrated e2e tests
  • components & utils
  • routes
  • server functions

refs:

• Vitest
• Playwright
• https://tanstack.com/router/latest/docs/framework/react/how-to/test-file-based-routing

[sd44]

I'd love to see implementations with Playwright or similar E2E tools. The official TanStack testing utilities often feel incomplete, especially when rigorously testing serverFn and routing logic in real network environments.