feat: add feature carousel hero + enable Stripe billing

- Replace static dashboard screenshot with interactive feature carousel
  9 slides: Dashboard, Mitglieder, Kurse, Finanzen, Veranstaltungen,
  Newsletter, Website, Buchungen, Dokumente
  Auto-advances every 6s, clickable sidebar + bottom tabs
  Virtual app UI rendered with shadcn components (no images needed)

- Enable Stripe test mode billing
  Add publishable key to .env.development, .env.production, docker-compose
  Add secret key to .env.development and docker-compose
  Add NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY to Dockerfile build args

apps/web/app/[locale]/(marketing)/page.tsx

@@ -1,4 +1,3 @@
-import Image from 'next/image';
 import Link from 'next/link';
 
 import {
@@ -38,6 +37,7 @@ import billingConfig from '~/config/billing.config';
 import pathsConfig from '~/config/paths.config';
 
 import { AnimateOnScroll } from './_components/animate-on-scroll';
+import { FeatureCarousel } from './_components/feature-carousel';
 
 function Home() {
   return (
@@ -72,16 +72,7 @@ function Home() {
                 className="bg-primary/10 absolute inset-0 -z-10 mx-auto max-w-3xl rounded-full blur-3xl"
                 aria-hidden="true"
               />
-              <Image
-                priority
-                className={
-                  'dark:border-primary/10 w-full rounded-2xl border border-gray-200 shadow-2xl'
-                }
-                width={3558}
-                height={2222}
-                src={`/images/dashboard.webp`}
-                alt={`MyEasyCMS Dashboard`}
-              />
+              <FeatureCarousel />
             </div>
           }
         />