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
d87fbb050f