feat: add action options for closing the application

lib/core/preferences/actions_at_closing.dart

@@ -0,0 +1,13 @@
+import 'package:hiddify/gen/translations.g.dart';
+
+enum ActionsAtClosing {
+  ask,
+  hide,
+  exit;
+
+  String present(TranslationsEn t) => switch (this) {
+        ask => t.settings.general.actionsAtClosing.askEachTime,
+        hide => t.settings.general.actionsAtClosing.hide,
+        exit => t.settings.general.actionsAtClosing.exit,
+      };
+}

lib/core/preferences/general_preferences.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/foundation.dart';
 import 'package:hiddify/core/app_info/app_info_provider.dart';
 import 'package:hiddify/core/model/environment.dart';
+import 'package:hiddify/core/preferences/actions_at_closing.dart';
 // import 'package:hiddify/core/model/region.dart';
 import 'package:hiddify/core/preferences/preferences_provider.dart';
 import 'package:hiddify/core/utils/preferences_utils.dart';
@@ -61,6 +62,13 @@ abstract class Preferences {
     "store_reviewed_by_user",
     false,
   );
+
+  static final actionAtClose = PreferencesNotifier.create<ActionsAtClosing, String>(
+    "action_at_close",
+    ActionsAtClosing.ask,
+    mapFrom: ActionsAtClosing.values.byName,
+    mapTo: (value) => value.name,
+  );
 }
 
 @Riverpod(keepAlive: true)