Adds firebase_core + firebase_analytics and a FirebaseAnalyticsBackend that
adapts the existing AnalyticsBackend interface to GA4. Kept in its own file
so the typed AnalyticsService and DebugAnalyticsBackend stay free of the
firebase dependency (unit tests never pull in platform channels).
Not yet wired into main() — that needs lib/firebase_options.dart from
flutterfire configure (owner step). All 161 tests still green.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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