Add per-core prerelease update selection

Replaces the global prerelease toggle with per-core prerelease options in both WPF and Avalonia update views, and persists selections via `CheckPreReleaseCoreTypes` in config. Update checks now resolve prerelease mode per selected core (including auto-check flow), and the sing-box max-version lock is removed to allow newer versions.

v2rayN/ServiceLib/Models/Configs/ConfigItems.cs

@@ -242,9 +242,9 @@ public class WebDavItem
 [Serializable]
 public class CheckUpdateItem
 {
-    public bool CheckPreReleaseUpdate { get; set; }
     public bool UpdateViaProxy { get; set; } = true;
     public List<string>? SelectedCoreTypes { get; set; }
+    public List<string>? CheckPreReleaseCoreTypes { get; set; }
 }
 
 [Serializable]

v2rayN/ServiceLib/Models/Dto/CheckUpdateModel.cs

@@ -3,6 +3,8 @@ namespace ServiceLib.Models.Dto;
 public partial class CheckUpdateModel : ReactiveObject
 {
     public bool? IsSelected { get; set; }
+    public bool IsCheckPreRelease { get; set; }
+    public bool ShowCheckPreRelease { get; set; }
     public ECoreType? CoreType { get; set; }
     [Reactive] public partial string? Remarks { get; set; }
     public string? FileName { get; set; }