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/payment/screens/payment_method_screen.dart

@@ -2,6 +2,7 @@ import 'package:dio/dio.dart';
 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/constants.dart';
 import '../../../core/theme/halo_tokens.dart';
@@ -53,7 +54,21 @@ class _PaymentMethodScreenState extends ConsumerState<PaymentMethodScreen> {
       _error = null;
     });
     final api = ref.read(apiClientProvider);
+    final analytics = ref.read(analyticsProvider);
     try {
+      // ⭐ Capture GA4 stitching identifiers BEFORE the POST so the backend can
+      // store them in product_metadata and replay them in the server-fired
+      // payment_confirmed (Measurement Protocol). The backend currently
+      // ignores unknown body fields — intentional; we send now, stitch later.
+      final appInstanceId = await analytics.appInstanceId();
+      final gaSessionId = await analytics.sessionId();
+      if (!mounted) return;
+
+      final analyticsIds = <String, dynamic>{
+        if (appInstanceId != null) 'app_instance_id': appInstanceId,
+        if (gaSessionId != null) 'ga_session_id': gaSessionId,
+      };
+
       final body = <String, dynamic>{
         'mode': draft.mode.value,
         'duration_minutes': draft.durationMinutes,
@@ -61,11 +76,27 @@ class _PaymentMethodScreenState extends ConsumerState<PaymentMethodScreen> {
         'is_first_session_discount': draft.isFirstSessionDiscount,
         'method': _selectedCode,
         if (draft.targetedMitraId != null) 'targeted_mitra_id': draft.targetedMitraId,
+        if (analyticsIds.isNotEmpty) 'analytics': analyticsIds,
       };
       final response = await api.post('/api/client/payment-requests/', data: body);
       final data = response['data'] as Map<String, dynamic>;
       final paymentId = data['id'] as String;
       ref.read(paymentDraftNotifierProvider.notifier).setPaymentId(paymentId);
+
+      // ⭐ payment_started fires AFTER the id is known. A targeted mitra means
+      // the returning/repeat funnel; otherwise activation.
+      final isRepeat = draft.targetedMitraId != null;
+      // ignore: discarded_futures
+      analytics.logPaymentStarted(
+        paymentRequestId: paymentId,
+        amount: draft.priceIDR!,
+        method: _selectedCode!,
+        funnel: isRepeat ? AnalyticsFunnel.repeat : AnalyticsFunnel.activation,
+        isRepeat: isRepeat,
+        productType: draft.mode.value,
+        durationMinutes: draft.durationMinutes!,
+      );
+
       if (!mounted) return;
       context.push('/payment/waiting/$paymentId');
     } on DioException catch (e) {
@@ -216,10 +247,20 @@ class _PaymentMethodScreenState extends ConsumerState<PaymentMethodScreen> {
                           _expandedGroupIds.remove(g.id);
                         }
                       }),
-                      onSelect: (code) => setState(() {
-                        _selectedCode = code;
-                        _error = null;
-                      }),
+                      onSelect: (code) {
+                        // Funnel step 9 — method chosen. Fire once per pick
+                        // (not on every rebuild).
+                        if (code != _selectedCode) {
+                          // ignore: discarded_futures
+                          ref
+                              .read(analyticsProvider)
+                              .logPaymentMethodSelect(method: code);
+                        }
+                        setState(() {
+                          _selectedCode = code;
+                          _error = null;
+                        });
+                      },
                     );
                   }).toList(),
                 );