Move the existing Atlas sign in modal handling from the `atlas-service` package to the `compass-generative-ai` package.
https://github.com/mongodb-js/compass/blob/ef3ebe9d0558f037bb0ee26991d62d5102f83bfb/packages/atlas-service/src/store/atlas-signin-store.ts#L23
https://github.com/mongodb-js/compass/blob/ef3ebe9d0558f037bb0ee26991d62d5102f83bfb/packages/atlas-service/src/components/ai-signin-modal.tsx 

We want all of the sign in and auth logic to stay in the atlas-service. Only the frontend here we want in one place since the current login is generative ai specific.

We're doing this as we're adding more Atlas Gen ai enable modals so down the line when we combine them it should be more straight forward.