feat(i18n): add Language preference and Save() to AppSettings

src/ClaudeDo.Ui/AppSettings.cs

@@ -7,6 +7,7 @@ public sealed class AppSettings
 {
     public string DbPath { get; set; } = "~/.todo-app/todo.db";
     public string SignalRUrl { get; set; } = "http://127.0.0.1:47821/hub";
+    public string Language { get; set; } = "";
 
     private static readonly string ConfigPath = Paths.Expand("~/.todo-app/ui.config.json");
 
@@ -27,4 +28,12 @@ public sealed class AppSettings
         }
         return new();
     }
+
+    public void Save()
+    {
+        var dir = Path.GetDirectoryName(ConfigPath);
+        if (!string.IsNullOrEmpty(dir)) Directory.CreateDirectory(dir);
+        var json = JsonSerializer.Serialize(this, new JsonSerializerOptions { WriteIndented = true });
+        File.WriteAllText(ConfigPath, json);
+    }
 }