feat(analytics): default GA to TXLookup property (G-19ZRJ9T0CX)

[jravinder]

Why

The Analytics component in app/components/Analytics.tsx is already wired into app/layout.tsx, but it only renders the gtag snippet when NEXT_PUBLIC_GA_ID is set in env. That env var was never set on Vercel, so production has been collecting zero analytics.

Change

• New DEFAULT_GA_ID = "G-19ZRJ9T0CX" constant — the TXLookup GA4 property created today (property 539004666 under hackathon-acf11 / accounts/3000023).
• Component now uses the env var if defined (override / per-environment), the default if not, and disables entirely if the env var is explicitly empty (NEXT_PUBLIC_GA_ID="" for local dev).

What happens on merge

• Vercel rebuilds on tag (deploy is tag-gated per current workflow), or trigger manually via workflow_dispatch.
• After the next deploy, page views start landing in the TXLookup property. Realtime view should show data within ~30 seconds of the first visit.

Verifying

After deploy:

ACCESS_TOKEN=$(gcloud auth application-default print-access-token)
curl -s -H "Authorization: Bearer $ACCESS_TOKEN" -H "x-goog-user-project: hackathon-acf11" \
  "https://analyticsdata.googleapis.com/v1beta/properties/539004666:runRealtimeReport" \
  -X POST -H "Content-Type: application/json" \
  -d '{"metrics":[{"name":"activeUsers"}]}'

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