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/otp_screen.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import '../../../core/analytics/analytics_service.dart';
 import '../../../core/api/api_client_provider.dart';
 import '../../../core/auth/auth_notifier.dart';
 import '../../../core/constants.dart';
@@ -75,6 +76,14 @@ class _OtpScreenState extends ConsumerState<OtpScreen> {
         if (_errorMessage != null && mounted) {
           setState(() => _errorMessage = null);
         }
+        // OTP verify resolved to a real identity — activation/repeat step 6.
+        final data = next.valueOrNull;
+        if (data is AuthAuthenticatedData || data is AuthNeedsDisplayNameData) {
+          // ignore: discarded_futures
+          ref
+              .read(analyticsProvider)
+              .logAuthComplete(AnalyticsUserType.verified);
+        }
       }
     });
 
@@ -152,6 +161,8 @@ class _OtpScreenState extends ConsumerState<OtpScreen> {
     final code = _readCode();
     if (code.length == _kOtpLength && !_autoSubmitted && _otpRequestId != null) {
       _autoSubmitted = true;
+      // ignore: discarded_futures
+      ref.read(analyticsProvider).logAuthOtpSubmit();
       ref.read(authProvider.notifier).verifyOtp(_otpRequestId!, code);
     }
   }