Add an option to check for update without proxy (#10090)

Checking for updates always dialed through the local proxy, which fails
exactly when an update may be needed most - when no working server is
available. Like the subscription-update menu, updating without the proxy
is now possible: the Check Update window gets an "Update via proxy"
toggle (persisted, on by default so existing behavior is unchanged),
honored by the check-only, update and geo-file flows, and by the
scheduled update check.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

v2rayN/ServiceLib/Models/Configs/ConfigItems.cs

@@ -243,6 +243,7 @@ public class WebDavItem
 public class CheckUpdateItem
 {
     public bool CheckPreReleaseUpdate { get; set; }
+    public bool UpdateViaProxy { get; set; } = true;
     public List<string>? SelectedCoreTypes { get; set; }
 }