LeadBot Headless test site

Same components, three integrations, all rendering published steps of one headless leadbot. Configure via .env (VITE_LEADBOT_DOMAIN_KEY, VITE_LEADBOT_LEADBOT_ID, VITE_LEADBOT_HOST, VITE_LEADBOT_API_HOST) or URL params (?domainKey=…&leadbotId=…&host=…&apiHost=…&baseColor=…&preview=1). Step ids are discovered from the boot fetch unless pinned.