Fix hardcoded Mock Server base URL

Mock URL copy uses a hardcoded :8000 port which breaks in production.\n\nFiles:\n- frontend/src/pages/MockServer.tsx\n\nTasks:\n- Use window.location.origin\n- Remove hardcoded port\n- Ensure copied URL works behind reverse proxies\n\nAcceptance Criteria:\n- Copied URL works in dev and production\n- No hardcoded port in generated URL