dsteem Modernization Plan -Phase 6

Day 6: I finished the test infrastructure overhaul. Mocha 5 → 11, nyc → c8 with a 70% line-coverage gate that CI enforces, Playwright running headless Chromium/Firefox/WebKit against the real bundled browser artifact, and — finally — the 6-year-old broken inspect() test got a proper fix using util.inspect.custom. Plus a tidy little env-gate system so live api.steemit.com + api.moecki.online calls don't make CI flaky.