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/register_screen.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.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/constants.dart';
 import '../../../core/theme/halo_tokens.dart';
@@ -222,8 +223,15 @@ class _RegisterScreenState extends ConsumerState<RegisterScreen> {
                         : 'kirim kode',
                 fullWidth: true,
                 onPressed: canSubmit
-                    ? () =>
-                        ref.read(authProvider.notifier).requestOtp(_e164Phone())
+                    ? () {
+                        // ignore: discarded_futures
+                        ref
+                            .read(analyticsProvider)
+                            .logAuthStart(AnalyticsAuthMethod.phone);
+                        ref
+                            .read(authProvider.notifier)
+                            .requestOtp(_e164Phone());
+                      }
                     : null,
               ),
               if (!fromProfile) ...[