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Extends the checkout pattern (inline red error text, refocus on submit if invalid) to all four newsletter-signup entry points. Two of them (WeeklyImpulsesHero's inline hero form on /newsletter, and /challenge's EmailCapture) turned out to be completely non-functional before this too — same static-markup-with-no-onSubmit issue as Newsletter.tsx/NewsletterModal.tsx had, just missed in the previous pass since they're separate components sharing only the visual pattern, not the code. Consolidated the shared email+consent+submit state (previously duplicated per-component) into useNewsletterSignup.ts, and pulled the plain email-format regex (previously duplicated in CheckoutContent.tsx and the subscribe route) into lib/email.ts as a single source of truth. /challenge's EmailCapture is now its own client component (app/challenge/components/EmailCapture.tsx) since its parent page is an async Server Component and can't hold form state itself. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>