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

docker-compose.local.yml

@@ -323,6 +323,7 @@ services:
         NEXT_PUBLIC_SUPABASE_URL: http://localhost:8000
         NEXT_PUBLIC_SUPABASE_PUBLIC_KEY: ${SUPABASE_ANON_KEY:-eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpc3MiOiJzdXBhYmFzZS1kZW1vIiwicm9sZSI6ImFub24iLCJleHAiOjE5ODM4MTI5OTZ9.CRXP1A7WOeoJeXxjNni43kdQwgnWNReilDMblYTn_I0}
         NEXT_PUBLIC_DEFAULT_LOCALE: de
+        NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY: 'pk_test_51SMbesKttnWb7SsFOR7cJ1jshdEaZiHmCflWndgLtL3gx1Cu8N4p5qxSJY8PHmpEJL8gf4VrqqX2Fr7pxJtQILUS00yYQ7Tx8V'
     restart: unless-stopped
     ports:
       - '3000:3000'
@@ -350,6 +351,9 @@ services:
       NEXT_PUBLIC_ENABLE_TEAM_ACCOUNTS_BILLING: 'false'
       NEXT_PUBLIC_ENABLE_PERSONAL_ACCOUNT_BILLING: 'false'
       NEXT_PUBLIC_ENABLE_NOTIFICATIONS: 'true'
+      # Stripe
+      NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY: 'pk_test_51SMbesKttnWb7SsFOR7cJ1jshdEaZiHmCflWndgLtL3gx1Cu8N4p5qxSJY8PHmpEJL8gf4VrqqX2Fr7pxJtQILUS00yYQ7Tx8V'
+      STRIPE_SECRET_KEY: 'sk_test_51SMbesKttnWb7SsFTjCsPZMlxVe3WjrsDnpLDAQehVdzSoDaWMFmc3hiZOTp2IAKB1cleMPIOW9GmEJHEkhazJsq00FEfTr6BI'
 
 volumes:
   supabase-db-data: