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/lib/features/auth/screens/display_name_screen.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
+import '../../../core/analytics/analytics_service.dart';
 import '../../../core/api/api_client_provider.dart';
 import '../../../core/auth/auth_notifier.dart';
 import '../../../core/theme/halo_tokens.dart';
@@ -35,6 +36,9 @@ class _DisplayNameScreenState extends ConsumerState<DisplayNameScreen> {
       final data = next.valueOrNull;
       if (data is AuthAnonymousData && !_routedAfterLogin) {
         _routedAfterLogin = true;
+        // Anonymous identity established — activation step 6.
+        // ignore: discarded_futures
+        ref.read(analyticsProvider).logAuthComplete(AnalyticsUserType.anonymous);
         _proceedAfterLogin();
       }
     });
@@ -82,12 +86,17 @@ class _DisplayNameScreenState extends ConsumerState<DisplayNameScreen> {
       return;
     }
 
+    // ignore: discarded_futures
+    ref.read(analyticsProvider).logVerifChoiceView();
     final choice = await VerifChoiceSheet.show(context);
     if (!mounted || choice == null) {
-      // User dismissed the sheet — let them tap Lanjut again to retry.
+      // User dismissed the sheet — let them tap Lanjut again to retry. No
+      // select event: the view→select gap is the abandonment signal.
       _routedAfterLogin = false;
       return;
     }
+    // ignore: discarded_futures
+    ref.read(analyticsProvider).logVerifChoiceSelect(choice);
     if (!mounted) return;
     await routeForVerifChoice(context, ref, choice);
   }