Only create cookie when needed

[coyotte508]

For example, when a product is added to cart. (I think that's the only thing)

There's no need to create a cookie before that.

The cookie should also be refreshed in layout.server.ts instead of hooks.server.ts, and only if cart is not empty. We can change the expiration from 1 year to 30 days as well.

Related to #48