Show monthly equivalent price for annual plans on checkout

When a product has both monthly and annual pricing, the checkout should display the annual plan's monthly equivalent cost alongside the total annual price. For example, if the annual plan is $120/year, show "$10/month, billed annually" so customers can directly compare it to the monthly plan. Currently only the raw annual price is shown, which makes it appear more expensive than the monthly option even when it's not — leading to lower annual plan conversions.