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);
+}