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/ServiceLib/Resx/ResUI.Designer.cs

@@ -870,6 +870,15 @@ namespace ServiceLib.Resx {
             }
         }
         
+        /// <summary>
+        ///   查找类似 Only Check 的本地化字符串。
+        /// </summary>
+        public static string menuCheckOnly {
+            get {
+                return ResourceManager.GetString("menuCheckOnly", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   查找类似 Check Update 的本地化字符串。
         /// </summary>
@@ -3934,7 +3943,7 @@ namespace ServiceLib.Resx {
         }
         
         /// <summary>
-        ///   查找类似 Check for pre-release updates 的本地化字符串。
+        ///   查找类似 Check for pre-release 的本地化字符串。
         /// </summary>
         public static string TbSettingsEnableCheckPreReleaseUpdate {
             get {