Add integration smoke tests to CI

I've created a basic GitHub action for the repo in https://github.com/divamgupta/diffusionbee-stable-diffusion-ui/pull/170. From there, it would be extremely valuable to add some basic smoke tests to make sure that the app's basic behavior works in each PR.

- Renders
- Each tab is present
- Clicking "Generate" works

I recommend [using Playwright](https://dev.to/azure/003-lets-automate-playwright-testing-with-github-actions-g1l) here. There's a good article on writing Electron tests with it here: https://blog.stranianelli.com/how-to-test-electronjs-app-english/

Further, I would recommend following [Testing Library's](https://testing-library.com/) lead here and try to use [`byRole`](https://playwright.dev/docs/next/api/class-locator#locator-get-by-role) queries to ensure we're following accessibility best practices.