added mysql connection

Server/DbConfig.cs

@@ -0,0 +1,37 @@
+using System.Text.Json;
+
+namespace Server;
+
+public class DbConfig
+{
+    private const string CONFIG_PATH = "config.json";
+
+    public string Username { get; set; } = "";
+    public string Password { get; set; } = "";
+    public string Server   { get; set; } = "";
+    public string Port     { get; set; } = ""; // new
+    public string Database { get; set; } = "";
+
+    public void Save()
+    {
+        var options = new JsonSerializerOptions { WriteIndented = true };
+        string json = JsonSerializer.Serialize(this, options);
+        File.WriteAllText(CONFIG_PATH, json);
+    }
+
+    public static DbConfig? Load()
+    {
+        if (!File.Exists(CONFIG_PATH))
+            return null;
+
+        try
+        {
+            string json = File.ReadAllText(CONFIG_PATH);
+            return JsonSerializer.Deserialize<DbConfig>(json);
+        }
+        catch
+        {
+            return null;
+        }
+    }
+}