Add 'Check Only' update action

Introduce a new "Check Only" feature: add CheckOnlyCmd to CheckUpdateViewModel with CheckOnlyTask that queries updates (via UpdateService.CheckHasUpdateOnly) for selected cores and reports results without performing updates. Wire up a new btnCheckOnly in both Desktop and Avalonia views and bind the command. Add localized menuCheckOnly entries to multiple .resx files and update ResUI.Designer. Also shorten the pre-release label to "Check for pre-release" in resource files.

v2rayN/v2rayN.Desktop/Views/CheckUpdateView.axaml

@@ -33,6 +33,12 @@
                     Margin="{StaticResource Margin4}"
                     HorizontalAlignment="Left" />
 
+                <Button
+                    x:Name="btnCheckOnly"
+                    Width="100"
+                    Margin="{StaticResource Margin4}"
+                    Content="{x:Static resx:ResUI.menuCheckOnly}" />
+
                 <Button
                     x:Name="btnCheckUpdate"
                     Width="100"

v2rayN/v2rayN.Desktop/Views/CheckUpdateView.axaml.cs

@@ -13,6 +13,7 @@ public partial class CheckUpdateView : ReactiveUserControl<CheckUpdateViewModel>
             this.OneWayBind(ViewModel, vm => vm.CheckUpdateModels, v => v.lstCheckUpdates.ItemsSource).DisposeWith(disposables);
 
             this.Bind(ViewModel, vm => vm.EnableCheckPreReleaseUpdate, v => v.togEnableCheckPreReleaseUpdate.IsChecked).DisposeWith(disposables);
+            this.BindCommand(ViewModel, vm => vm.CheckOnlyCmd, v => v.btnCheckOnly).DisposeWith(disposables);
             this.BindCommand(ViewModel, vm => vm.CheckUpdateCmd, v => v.btnCheckUpdate).DisposeWith(disposables);
         });
     }