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/chat/widgets/pricing_bottom_sheet.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import '../../../core/analytics/analytics_service.dart';
 import '../../../core/chat/chat_opening_provider.dart';
 import '../../../core/chat/session_closure_notifier.dart';
 import '../../../core/constants.dart';
@@ -85,6 +86,10 @@ class _PricingBottomSheetState extends ConsumerState<PricingBottomSheet> {
   }
 
   void _onConfirm(PriceTier tier) {
+    // ignore: discarded_futures
+    ref.read(analyticsProvider).logChatExtensionRequested(
+          sessionId: widget.extensionSessionId,
+        );
     Navigator.of(context).pop();
     ref.read(sessionClosureProvider.notifier).requestExtension(
           widget.extensionSessionId,