#32069107 A4A: Enable referral cobranded checkout flag and refine logo options

Fixes https://linear.app/a8c/issue/A4A-2202/janitorial-enable-the-a4a-referral-cobranded-checkout-feature-flag-on

Proposed Changes

• Enable the a4a-referral-cobranded-checkout feature flag for A4A stage and production environments.
• Refine the referral logo radio options so the second option label is dynamic:
  • “Use a different logo for this referral” when a profile logo exists.
  • “Use this logo” when only an agency referrals logo exists.
  • “Upload my logo” when no logo exists yet.

Why are these changes being made?

• Roll out the cobranded referral checkout experience beyond dev/horizon so agencies can use the same flow in stage and production.
• Make the referral logo choices clearer and more contextual, especially when an existing logo is already configured.

Testing Instructions

• In the stage environment, verify that the cobranded referral checkout flow is available when requesting client payment.
• In the production environment, verify that the cobranded referral checkout flow is available when requesting client payment.
• With different logo states (profile logo only, referrals logo only, no logo), verify that the referral logo radio labels match the expected copy.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the “[Status] Needs Privacy Updates” label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?