Make newsletter signup confirmation consistent and casual across all 4 forms

Same short "check your inbox" message everywhere instead of each form
having its own success wording (they'd drifted since each is a
separately-coded component, not a shared one).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/challenge/components/EmailCapture.tsx

@@ -17,7 +17,7 @@ export function EmailCapture({ buttonLabel = "Challenge starten" }: { buttonLabe
     useNewsletterSignup("challenge");
 
   if (status === "success") {
-    return <p className="text-[1rem] text-[#222221] font-medium">Danke! Du bist jetzt für den Newsletter angemeldet.</p>;
+    return <p className="text-[1rem] text-[#222221] font-medium">Fast geschafft! Schau kurz in dein Postfach – da wartet schon eine Mail von uns.</p>;
   }
 
   return (