Add alpha session gate with cookie auth and login UI.

src/HSchool.Server/Net/ClientRegistry.cs

@@ -25,4 +25,22 @@ internal sealed class ClientRegistry
     public GameClient? Find(uint playerId) => _clients.GetValueOrDefault(playerId);
 
     public void Remove(uint playerId) => _clients.TryRemove(playerId, out _);
+
+    public bool IsUserNameOnline(string normalizedUserName)
+    {
+        foreach (var client in _clients.Values)
+        {
+            if (client.UserName is null)
+            {
+                continue;
+            }
+
+            if (string.Equals(client.NormalizedUserName, normalizedUserName, StringComparison.OrdinalIgnoreCase))
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
 }