feat(my-account): add v2 prototype Reader Account Customization admin (Phase 10)

[thomasguillot]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Adds a new Reader Account Customization tab to the Audience wizard, between Configuration and Checkout & Payment. Out-of-roadmap addition to the My Account v2 prototype (umbrella tracker #4679) — the first v2 surface that lives in WP-admin rather than /my-account/. Publishers asked to see how the admin will potentially look, so this gives them a clickable preview of where the v2 reader-facing customisation controls would surface.

The screen is a pure-demo: local React useState only, no REST roundtrip, no persistence, Save button is a no-op. Three two-column sections (title + description left, controls right — same shape as the Access Control edit page):

• Branding — logo upload + helper copy.
• Newsletters — page title + page description.
• Account & Billing — Terminology toggle (Subscription / Membership / Custom; Custom reveals stacked singular + plural inputs) + cancel-recurring-donation message + billing/invoice footer.

Productionisation (real persistence, terminology actually driving v2 prototype labels, real Stripe/WC wiring on inputs) is documented in the Phase 10 devlog entry.

How to test the changes in this Pull Request:

1. Pull this branch and run ` npm run build ` (or ` npm start ` for watch).
2. As an administrator, visit ` /wp-admin/admin.php?page=newspack-audience#/reader-account-customization `.
3. Confirm the tab order is Configuration → Reader Account Customization → Checkout & Payment.
4. Confirm the page renders three sections (Branding, Newsletters, Account & Billing) with the two-column layout (title + description on the left, controls on the right) and a divider between each.
5. Confirm vertical spacing inside each section is consistent: 32px between every control (no ragged 16px gaps from legacy BaseControl margins).
6. Click Custom in the Terminology toggle — confirm two stacked inputs (singular, plural) appear with placeholders ` e.g. Patronage ` / ` e.g. Patronages `. Toggle back to Subscription / Membership and confirm they hide.
7. Type into each field; confirm state holds while you stay on the tab and resets if you navigate away and back (pure-demo, no persistence by design).
8. Click Save Settings — confirm it's a no-op (no network request, no state change).
9. Visit the same URL as a non-admin or logged-out user — confirm the tab renders normally (no admin-gating on this surface; it's part of the existing wizard which already requires admin caps).
10. Confirm the wizard's other tabs (Configuration, Checkout & Payment) still work as before.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?