Introduce a new `UpdateSubscriptionQuantity` Stripe action to:
- preview prorated due-now and next-cycle recurring costs
- update subscription item quantity with proration invoicing
- revert quantity and void invoice when payment is not completed
Wire the flow into the Livewire subscription actions UI with a new adjust-limit modal,
price preview loading, and confirmation-based updates. Also refactor the subscription
management section layout and fix modal confirmation behavior for temporary 2FA bypass.
Add `Subscription::billingInterval()` helper and comprehensive Pest coverage for
quantity updates, preview calculations, failure/revert paths, and billing interval logic.
