feat(client/analytics): GA4 funnel instrumentation + unified home CTA

Add Firebase Analytics (GA4) funnel tracking to client_app:
- AnalyticsService typed wrapper (enum-gated, no PII) + analyticsProvider
- FirebaseAnalyticsObserver on GoRouter (screen_name via nameExtractor)
- user_id = customer UUID, user_type property, set on auth resolve/upgrade
- funnel events: curhat_start, curhat_repeat_start, auth_*, onboarding_usp_view,
  payment_view, payment_method_select, payment_started, pairing_matched/no_bestie
- bottom-sheet events: verif_choice_view/select, bestie_choice_view/select,
  extension_offer_view, chat_extension_requested
- payment_started carries app_instance_id + ga_session_id in the
  /payment-requests body for future server-side stitching (backend ignores)
- curhat_mode_pick screen name disambiguates the chat/call mode picker
  (/payment/method-pick) from the payment-channel picker (/payment/method)
- unify both home CTAs to "Aku Mau Curhat"

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client_app/pubspec.yaml

@@ -14,6 +14,9 @@ dependencies:
   # Firebase (Messaging only — Auth dropped in Phase 3.4, self-managed JWT now)
   firebase_core: ^3.12.1
   firebase_messaging: ^15.2.5
+  # GA4 funnel analytics (no PII — see requirement/analytics-funnel-plan.md).
+  # ^11.x is the firebase_core ^3.x-compatible major.
+  firebase_analytics: ^11.4.4
 
   # Social login (kept — buttons gated server-side via /api/shared/auth-providers
   # until the corresponding OAuth env vars are set on the backend)