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/home/home_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/auth/auth_notifier.dart';
 import '../../core/auth/onboarding_intent_provider.dart';
 import '../../core/availability/mitra_availability_notifier.dart';
@@ -81,6 +82,11 @@ class _HomeScreenState extends ConsumerState<HomeScreen> with WidgetsBindingObse
   /// CTA path for SHomeReturning. Returning users get the bestie-choice sheet
   /// when they have prior history, otherwise jump to the new-payment shell.
   Future<void> _onCurhatBestieBaruPressed(BuildContext context) async {
+    // Returning user starting a fresh curhat (repeat funnel). The
+    // bestie_reselect sub-event fires later from the history list if they pick
+    // a known bestie; this marks the top of the repeat funnel.
+    // ignore: discarded_futures
+    ref.read(analyticsProvider).logCurhatRepeatStart();
     bool hasHistory;
     try {
       hasHistory = await ref.read(bestieHistoryHasItemsProvider.future);
@@ -89,6 +95,8 @@ class _HomeScreenState extends ConsumerState<HomeScreen> with WidgetsBindingObse
     }
     if (!context.mounted) return;
     if (hasHistory) {
+      // ignore: discarded_futures
+      ref.read(analyticsProvider).logBestieChoiceView();
       await BestieChoiceSheet.show(context);
       return;
     }
@@ -101,6 +109,9 @@ class _HomeScreenState extends ConsumerState<HomeScreen> with WidgetsBindingObse
   /// (the call-sign check); display_name_screen kicks off `loginAnonymous`
   /// and pushes into the verif-choice sheet.
   void _onAkuMauCurhatPressed(BuildContext context) {
+    // Top of the activation funnel — fresh user tapping the primary CTA.
+    // ignore: discarded_futures
+    ref.read(analyticsProvider).logCurhatStart(entryPoint: 'home_primary');
     context.push('/auth/display-name');
   }
 
@@ -172,7 +183,7 @@ class _SHome1stView extends ConsumerWidget {
               const _GreetingSubtitle(),
               const SizedBox(height: 32),
               _PrimaryCTA(
-                label: 'aku mau curhat',
+                label: 'Aku Mau Curhat',
                 enabled: mitraAvailable,
                 onPressed: onCTA,
               ),
@@ -395,7 +406,7 @@ class _SHomeReturningView extends ConsumerWidget {
         activeSessionAsync.when(
           loading: () => const Center(child: CircularProgressIndicator()),
           error: (_, __) => _PrimaryCTA(
-            label: 'curhat sama bestie baru',
+            label: 'Aku Mau Curhat',
             enabled: mitraAvailable,
             onPressed: onCTA,
           ),
@@ -415,7 +426,7 @@ class _SHomeReturningView extends ConsumerWidget {
               );
             }
             return _PrimaryCTA(
-              label: 'curhat sama bestie baru',
+              label: 'Aku Mau Curhat',
               enabled: mitraAvailable,
               onPressed: onCTA,
             );