Add live-metrics dashboard + harden verification

Details

- New test/ga4_dashboard.mjs queries the GA4 Data API for realtime
  active users plus 24h/7d/30d users/sessions/page-views/events and
  top events/pages, emitting a timestamped HTML dashboard
  (test/ga4_dashboard_<ts>.html plus _latest.html).
- Skip-with-warn when GA4_PROPERTY_ID / GA4_SERVICE_ACCOUNT_JSON
  are unset; non-zero exit when creds set but zero events in 30d
  or on auth/config errors.
- Wire into frontend/scripts/local.sh and .github/workflows/ci.yml;
  CI uploads dashboard + verification report + screenshots as the
  "ga4-dashboard" artifact.
- Rewrite test/verify_google_analytics.mjs to run 11 rigorous
  assertions per page (inline snippet, gtag.js load, runtime gtag/
  dataLayer state, v=2 collect hits, tid/cid/page_view, console
  cleanliness) across / and /v1/, with HTML report.
- Doc updates: README "GA4 Live-Metrics Dashboard" section,
  .env.runtime template placeholders, .gitignore for build outputs.
Co-authored-by: Cursor <cursoragent@cursor.com>

Files changed (9)