Display Savings and Monthly Equivalent on Subscription Plans

Add an option to display savings information for quarterly and annual subscription plans. This could show either:

• The monthly equivalent price (e.g. £600/month equivalent instead of only showing £3,600/year)

• The percentage or monetary discount compared to monthly billing (e.g. Save 30%)

This helps clients quickly understand the value of longer-term plans and makes plan comparisons easier during checkout.