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/router.dart

@@ -1,6 +1,8 @@
+import 'package:firebase_analytics/firebase_analytics.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/auth/auth_notifier.dart';
 import 'core/auth/onboarding_intent_provider.dart';
 import 'features/auth/screens/display_name_screen.dart';
@@ -54,12 +56,62 @@ class RouterNotifier extends ChangeNotifier {
 
 final routerProvider = Provider<GoRouter>((ref) => buildRouter(ref));
 
+/// Maps a GoRoute path template (`Route.settings.name`) to a stable
+/// `screen_name`. Keyed on the *template* (e.g. `/chat/session/:sessionId`)
+/// so path params are never part of the logged name. Routes absent here are
+/// dropped (return null → observer skips the screen_view).
+const _screenNameByRoute = <String, AnalyticsScreen>{
+  '/splash': AnalyticsScreen.splash,
+  '/auth/display-name': AnalyticsScreen.authDisplayName,
+  '/auth/register': AnalyticsScreen.authRegister,
+  '/auth/otp': AnalyticsScreen.authOtp,
+  '/auth/set-name': AnalyticsScreen.authSetName,
+  '/auth/force-register': AnalyticsScreen.authForceRegister,
+  '/onboarding/verif/usp': AnalyticsScreen.onboardingUspVerified,
+  '/onboarding/anon/usp': AnalyticsScreen.onboardingUspAnon,
+  '/onboarding/notif-gate': AnalyticsScreen.onboardingNotifGate,
+  '/home': AnalyticsScreen.home,
+  '/profile': AnalyticsScreen.profile,
+  '/payment/entry': AnalyticsScreen.paymentEntry,
+  '/payment/discount-paywall': AnalyticsScreen.paymentDiscountPaywall,
+  '/payment/method-pick': AnalyticsScreen.curhatModePick,
+  '/payment/duration-pick': AnalyticsScreen.paymentDurationPick,
+  '/payment/method': AnalyticsScreen.paymentMethod,
+  '/payment/waiting/:paymentId': AnalyticsScreen.paymentWaiting,
+  '/payment/expired/:paymentId': AnalyticsScreen.paymentExpired,
+  '/chat/searching': AnalyticsScreen.chatSearching,
+  '/chat/found': AnalyticsScreen.chatFound,
+  '/chat/no-bestie': AnalyticsScreen.chatNoBestie,
+  '/chat/waiting-targeted/:mitraId': AnalyticsScreen.chatWaitingTargeted,
+  '/chat/session/:sessionId': AnalyticsScreen.chatSession,
+  '/chat/thank-you': AnalyticsScreen.chatThankYou,
+  '/chat/aktif': AnalyticsScreen.chatTabAktif,
+  '/chat/pembayaran': AnalyticsScreen.chatTabPembayaran,
+  '/chat/selesai': AnalyticsScreen.chatTabSelesai,
+  '/chat/transcript/:sessionId': AnalyticsScreen.chatTranscript,
+  '/bestie/history': AnalyticsScreen.bestieHistory,
+};
+
+/// `nameExtractor` for [FirebaseAnalyticsObserver]. GoRouter sets
+/// `Route.settings.name` to the route's path template, so this strips path
+/// params (`:sessionId` etc.) by construction.
+String? _screenNameFor(RouteSettings settings) {
+  final name = settings.name;
+  if (name == null) return null;
+  return _screenNameByRoute[name]?.value;
+}
+
 GoRouter buildRouter(Ref ref) {
   final notifier = RouterNotifier(ref);
+  final analyticsObserver = FirebaseAnalyticsObserver(
+    analytics: FirebaseAnalytics.instance,
+    nameExtractor: _screenNameFor,
+  );
 
   return GoRouter(
     initialLocation: kThemePreviewEnabled ? '/_theme_preview' : '/splash',
     refreshListenable: notifier,
+    observers: [analyticsObserver],
     redirect: (context, state) {
       // Theme preview is dev-only and intentionally bypasses auth + onboarding
       // gates so it can be opened on any device build.