`NewsletterSignupCard` — feature flag + swap in `EmailSignUpWrapper`

[georgerichmond]

Summary

Wire NewsletterSignupCard.island.tsx into the existing EmailSignUpWrapper.island.tsx behind a feature flag. The old component remains untouched and is the default. This ticket makes the new component visible in production for the first time.

Parent epic: guardian/newsletters-nx#583

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Tasks

• Add a feature flag for the new component
• Update EmailSignUpWrapper.island.tsx to conditionally render NewsletterSignupCard.island.tsx when the flag is enabled
• Verify the old component still renders correctly when the flag is off
• Smoke test both paths in CODE

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Out of scope

• No already-subscribed state
• No preview modal
• No frequency logic changes
• No deletion of EmailSignup.tsx or SecureSignup.island.tsx — these stay until the flag is fully rolled out

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Design reference

https://www.figma.com/design/ucERNBXZ9CAObsjnVWqgEi/Newsletter-Sign-Up-Component?node-id=1492-7289&t=9vObuzDe6UjYnHbv-11

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After this one we have frequency logic, preview modal and already-subscribed still to cover — which would you like next?