revert HEAD timeout 5000 → 2000 (matches original)

I bumped this to 5s during the springfiles rewrite without flagging it.
Original was 2s. Live springfiles HEADs respond in ~80ms so 2s is fine.

Author: Licho1

Zero-K.info/AppCode/ResourceLinkProvider.cs

@@ -161,7 +161,7 @@ static bool HeadCheck(string url, int length)
             try
             {
                 var wr = (HttpWebRequest)WebRequest.Create(url);
-                wr.Timeout = 5000;
+                wr.Timeout = 2000;
                 wr.Method = "HEAD";
                 using (var res = wr.GetResponse()) return res.ContentLength == length;
             }