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/onboarding/screens/usp_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/theme/halo_tokens.dart';
 import '../../../core/theme/widgets/widgets.dart';
 import '../usp_seen_provider.dart';
@@ -11,11 +12,30 @@ import '../usp_seen_provider.dart';
 ///
 /// `verified` ➞ USP → OTP (`/auth/register`).
 /// `anonymous` ➞ USP → `/payment/method-pick`.
-class UspScreen extends ConsumerWidget {
+class UspScreen extends ConsumerStatefulWidget {
   final bool verified;
 
   const UspScreen({super.key, required this.verified});
 
+  @override
+  ConsumerState<UspScreen> createState() => _UspScreenState();
+}
+
+class _UspScreenState extends ConsumerState<UspScreen> {
+  @override
+  void initState() {
+    super.initState();
+    // Activation funnel step 7 — fire on view (not teardown). One-shot:
+    // initState runs once per screen instance.
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      if (!mounted) return;
+      // ignore: discarded_futures
+      ref
+          .read(analyticsProvider)
+          .logOnboardingUspView(verified: widget.verified);
+    });
+  }
+
   static const _cards = [
     _UspCard(
       icon: Icons.bolt_outlined,
@@ -40,7 +60,7 @@ class UspScreen extends ConsumerWidget {
   ];
 
   @override
-  Widget build(BuildContext context, WidgetRef ref) {
+  Widget build(BuildContext context) {
     return Scaffold(
       appBar: AppBar(
         title: const Padding(
@@ -94,7 +114,7 @@ class UspScreen extends ConsumerWidget {
               HaloButton(
                 label: 'aku ngerti, lanjut',
                 fullWidth: true,
-                onPressed: () => _onContinue(context, ref),
+                onPressed: () => _onContinue(context),
               ),
             ],
           ),
@@ -103,12 +123,12 @@ class UspScreen extends ConsumerWidget {
     );
   }
 
-  Future<void> _onContinue(BuildContext context, WidgetRef ref) async {
+  Future<void> _onContinue(BuildContext context) async {
     // Persist the local + server flag before leaving — next time the user
     // hits VerifChoice, this screen is skipped.
     await ref.read(uspSeenProvider.notifier).markSeen();
     if (!context.mounted) return;
-    if (verified) {
+    if (widget.verified) {
       context.push('/auth/register');
     } else {
       context.push('/payment/method-pick');