feat: Implement Firebase push notifications with support for flavor-specific configurations.

lib/features/notifications/data/notification_repository.dart

@@ -73,6 +73,39 @@ class NotificationRepository {
       );
     }
   }
+
+  /// Register FCM token with backend.
+  /// POST /notifications/fcm-token
+  Future<void> registerFcmToken(String token, String device) async {
+    try {
+      await _dio.post('/notifications/fcm-token', data: {
+        'token': token,
+        'device': device,
+      });
+    } on DioException catch (e) {
+      if (e.error is ApiException) throw e.error as ApiException;
+      throw ApiException(
+        message: e.message ?? 'Failed to register FCM token',
+        statusCode: e.response?.statusCode,
+      );
+    }
+  }
+
+  /// Remove FCM token from backend.
+  /// DELETE /notifications/fcm-token
+  Future<void> removeFcmToken(String token) async {
+    try {
+      await _dio.delete('/notifications/fcm-token', data: {
+        'token': token,
+      });
+    } on DioException catch (e) {
+      if (e.error is ApiException) throw e.error as ApiException;
+      throw ApiException(
+        message: e.message ?? 'Failed to remove FCM token',
+        statusCode: e.response?.statusCode,
+      );
+    }
+  }
 }
 
 class NotificationsResponse {