`NewsletterSignup.island.tsx` — signed in flow

[georgerichmond]

Summary

Implement the signed-in flow for NewsletterSignup.island.tsx, including matching Storybook coverage.

Parent epic: @guardian/dotcom-rendering/issues/15677

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Tasks

• Implement the signed-in state for the newsletter signup component
• Show the Sign up button in the signed-in experience
• Display the newsletter privacy message underneath the signup card
• Show the success state after signup, including:
  • You're signed up
  • the newsletter name and frequency
  • a Browse more newsletters link
• Reuse the existing newsletter signup tracking pattern from SecureSignup.island.tsx / newsletter-sign-up-requests.ts so the new component reports the same interaction lifecycle.
• Add Storybook coverage for the signed-in state and success state

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Suggested component structure

• NewsletterSignup.island.tsx as the top-level auth-aware wrapper
• Shared NewsletterSignupCard / card shell for the outer chrome
• Shared NewsletterSignupSuccessConfirmation for the success state
• Shared NewsletterPrivacyMessage beneath the card
• Signed-in state can be kept as a small focused view inside the wrapper

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Reuse

Existing request and tracking logic can likely be reused from SecureSignup.island.tsx, including:

• useAuthStatus
• postFormData
• buildFormData
• sendTracking

The privacy message placement pattern already exists in:

• ContributionsEpicNewsletterSignup.tsx
• NewsletterSignupLayout.tsx

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Notes

• The Preview latest CTA is intentionally out of scope here and will be added in a later ticket.

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Out of scope

• Signed-out email entry flow
• Validation errors
• Active-field reveal behavior for logged-out users
• Feature flag work
• Preview latest button