add firebase, add screen flow

lib/core/services/analytics_service.dart

@@ -0,0 +1,88 @@
+import 'package:firebase_analytics/firebase_analytics.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+
+/// Firebase Analytics service for tracking user events across the app.
+///
+/// Usage:
+/// ```dart
+/// // Log add to cart event
+/// AnalyticsService.logAddToCart(
+///   productId: 'SKU123',
+///   productName: 'Gạch men 60x60',
+///   price: 150000,
+///   quantity: 2,
+/// );
+/// ```
+class AnalyticsService {
+  AnalyticsService._();
+
+  static final FirebaseAnalytics _analytics = FirebaseAnalytics.instance;
+
+  /// Get the analytics instance for NavigatorObserver
+  static FirebaseAnalytics get instance => _analytics;
+
+  /// Get the observer for automatic screen tracking in GoRouter
+  static FirebaseAnalyticsObserver get observer => FirebaseAnalyticsObserver(
+        analytics: _analytics,
+        nameExtractor: (settings) {
+          // GoRouter uses the path as the route name
+          final name = settings.name;
+          if (name != null && name.isNotEmpty && name != '/') {
+            return name;
+          }
+          return settings.name ?? '/';
+        },
+        routeFilter: (route) => route is PageRoute,
+      );
+
+  /// Log screen view manually
+  static Future<void> logScreenView({
+    required String screenName,
+    String? screenClass,
+  }) async {
+    try {
+      await _analytics.logScreenView(
+        screenName: screenName,
+        screenClass: screenClass,
+      );
+      debugPrint('📊 Analytics: screen_view - $screenName');
+    } catch (e) {
+      debugPrint('📊 Analytics error: $e');
+    }
+  }
+
+  /// Log add to cart event
+  ///
+  /// [productId] - Product SKU or ID
+  /// [productName] - Product display name
+  /// [price] - Unit price in VND
+  /// [quantity] - Quantity added
+  /// [category] - Optional product category
+  static Future<void> logAddToCart({
+    required String productId,
+    required String productName,
+    required double price,
+    required int quantity,
+    String? brand,
+  }) async {
+    try {
+      await _analytics.logAddToCart(
+        currency: 'VND',
+        value: price * quantity,
+        items: [
+          AnalyticsEventItem(
+            itemId: productId,
+            itemName: productName,
+            price: price,
+            quantity: quantity,
+            itemBrand: brand,
+          ),
+        ],
+      );
+      debugPrint('📊 Analytics: add_to_cart - $productName x$quantity');
+    } catch (e) {
+      debugPrint('📊 Analytics error: $e');
+    }
+  }
+}