Add on-blur email validation to every newsletter signup form

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>

app/checkout/components/CheckoutContent.tsx

@@ -19,6 +19,7 @@ import { dispatchAuthChanged } from "../../lib/auth";
 import { readCheckoutDraft, writeCheckoutDraft, clearCheckoutDraft } from "../../lib/checkoutDraft";
 import { normalizeVatId, isValidVatId } from "../../lib/vatId";
 import { computeExemptTotals, destinationCountry, isExemptionEligibleCountry } from "../../lib/vatExemption";
+import { validateEmailFormat } from "../../lib/email";
 import type { ShippingMethod, ShippingCountry, PaymentMethod, TrustBadge, ShippingSettings } from "../../lib/payload";
 import type { CustomerProfile } from "../../lib/customerAuth";
 
@@ -44,11 +45,6 @@ function validateRequired(label: string, value: string): string {
   return value.trim() ? "" : `${label} ist erforderlich.`;
 }
 
-function validateEmailFormat(value: string): string {
-  if (!value.trim()) return "E-Mail-Adresse ist erforderlich.";
-  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(value) ? "" : "Bitte eine gültige E-Mail-Adresse angeben.";
-}
-
 function validateZip(value: string, country: string, plzDigitsMap: Record<string, number>): string {
   if (!value.trim()) return "PLZ ist erforderlich.";
   const digits = plzDigitsMap[country] ?? 4;