better visual coherence

frontend/lib/modules/current_trip_save_button.dart

@@ -1,4 +1,5 @@
 
+import 'package:anyway/main.dart';
 import 'package:anyway/structs/trip.dart';
 import 'package:auto_size_text/auto_size_text.dart';
 import 'package:flutter/material.dart';
@@ -8,6 +9,13 @@ Widget saveButton(Trip trip) => ElevatedButton(
   onPressed: () async {
     SharedPreferences prefs = await SharedPreferences.getInstance();
     trip.toPrefs(prefs);
+    rootScaffoldMessengerKey.currentState!.showSnackBar(
+      SnackBar(
+        content: Text('Trip saved'),
+        duration: Duration(seconds: 2),
+        dismissDirection: DismissDirection.horizontal
+      )
+    );
   },
   child: SizedBox(
     width: 100,