Update newsletter/brand copy, hide VAT section without a vatId, relabel Lebensuhr breadcrumb

- Newsletter panel + modal: new title/description copy, weekly cadence wording
- Brand mentions ("einfach-produktiv") switched to "einfach produktiv."
- Impressum: Umsatzsteuer section (heading + TOC entry) only renders when vatId is set
- /lebensuhr breadcrumb eyebrow now reads "Lebensuhr" instead of "7-Tage-Challenge"

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

app/lebensuhr/page.tsx

@@ -152,7 +152,7 @@ export default async function ChallengePage() {
               <span>›</span>
               <Link href="/#werkzeuge" className="hover:text-brand transition-colors">Werkzeuge</Link>
               <span>›</span>
-              <span className="text-text-primary">7-Tage-Challenge</span>
+              <span className="text-text-primary">Lebensuhr</span>
             </p>
 
             {/* Everything else — centered in the remaining vertical space,