fix: isolate dev preflight environment

[leoncheng57]

Summary

• make the dev env file selectable with DCA_ENV_FILE while preserving ./.env as the default
• run dev preflight tests in an isolated temporary repo with a conflicting root .env
• assert no-auth and Basic-auth checks each reach the mock exactly once

Verification

• npm run typecheck
• npm test
• npm run build
• locked CI=1 npm run test:e2e (120 passed, 1 skipped; one unrelated test passed on retry)
• git diff --check