feat: integrate Firebase Analytics and configure navigation observer in app router

lib/core/analytics/firebase_analytics_config.dart

@@ -0,0 +1,13 @@
+import 'package:firebase_analytics/firebase_analytics.dart';
+
+/// Firebase Analytics feeds the linked GA4 property (`G-DHR81R6Z63` on web,
+/// native app streams for iOS/Android). One instance is reused across the app
+/// for events, and one observer tracks GoRouter screen transitions.
+class FirebaseAnalyticsConfig {
+  FirebaseAnalyticsConfig._();
+
+  static final FirebaseAnalytics instance = FirebaseAnalytics.instance;
+
+  static final FirebaseAnalyticsObserver navigationObserver =
+      FirebaseAnalyticsObserver(analytics: instance);
+}

lib/routing/app_router.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 'package:real_estate_mobile/core/analytics/firebase_analytics_config.dart';
 import 'package:real_estate_mobile/core/analytics/umami_analytics.dart';
 import 'package:real_estate_mobile/core/constants/app_colors.dart';
 import 'package:real_estate_mobile/core/widgets/app_shell.dart';
@@ -45,7 +46,10 @@ final routerProvider = Provider<GoRouter>((ref) {
   return GoRouter(
     initialLocation: '/splash',
     refreshListenable: authRefreshNotifier,
-    observers: [UmamiAnalytics.navigationObserver],
+    observers: [
+      UmamiAnalytics.navigationObserver,
+      FirebaseAnalyticsConfig.navigationObserver,
+    ],
     redirect: (context, state) {
       final authState = ref.read(authProvider);
       final isAuthenticated = authState.status == AuthStatus.authenticated;