Refactor HTTPInterceptor and add CustomSnackBar helper

Refactor HTTPInterceptor to handle error responses and add a CustomSnackBar
helper to display snack bars. This will improve error handling and user
feedback in the application.

lib/navigation/navigation_service.dart

@@ -0,0 +1,7 @@
+import 'package:flutter/material.dart';
+
+class NavigationService {
+  static GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();
+  static GlobalKey<ScaffoldMessengerState>? snackbarKey =
+      GlobalKey<ScaffoldMessengerState>();
+}