Fix invisible Stripe pay button, add brand favicon/OG image, navbar 3x3-System CTA, blog hero crop

- PaymentStep: bg-brand-primary was never a real Tailwind class, leaving
  the "Jetzt bezahlen" button unstyled/invisible; switched to bg-brand
  with dark text, matching every other CTA on the site.
- New app/icon.svg + app/apple-icon.png + app/favicon.ico: brand-yellow
  rounded square with a bold italic "e", replacing the Next.js default.
- New public/og-image.png (logo on the site's cream background) wired in
  as the SEO fallback default OG image/description/title in payload.ts.
- Navbar CTA: "Klarheitskompass" -> "Mein 3x3-System", now links to
  /3x3-system (page not built yet).
- Blog overview hero image anchored to the top instead of center-cropped.

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

app/components/Navbar.tsx

@@ -519,10 +519,10 @@ export function Navbar({ singleActiveProduct }: { singleActiveProduct: boolean }
                 Newsletter
               </button>
               <Link
-                href="/lebensuhr"
+                href="/3x3-system"
                 className="px-6 py-4 rounded-sm bg-brand text-h4 font-bold text-text-primary whitespace-nowrap hover:bg-brand-hover active:scale-[0.97] transition-all"
               >
-                Klarheitskompass
+                Mein 3x3-System
               </Link>
             </div>
 
@@ -668,11 +668,11 @@ export function Navbar({ singleActiveProduct }: { singleActiveProduct: boolean }
                   Newsletter
                 </button>
                 <Link
-                  href="/lebensuhr"
+                  href="/3x3-system"
                   onClick={closeMobile}
                   className="md:hidden min-h-11 flex items-center justify-center px-6 py-4 rounded-sm bg-brand text-h4 font-bold text-text-primary hover:bg-brand-hover active:scale-[0.97] transition-all"
                 >
-                  Klarheitskompass
+                  Mein 3x3-System
                 </Link>
               </motion.div>
             </div>