GA4 verifier waits for 'load', not 'networkidle0'

Details

The GA4 navigation wait used networkidle0, which requires zero in-flight
requests for 500ms. The site keeps the network busy after load with
background Supabase data/image fetches and config reads, so networkidle0
is flaky in CI and was timing out at 30s (blocking deploys), unrelated
to page changes.
Switch the navigation wait to 'load'. GA firing is still asserted
directly afterwards via the collect-hit wait (firstHitPromise), so test
coverage is unchanged; it just no longer hinges on strict network idle.

Files changed (1)