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);
+    }
 }

tests/ClaudeDo.Ui.Tests/AppSettingsTests.cs

@@ -0,0 +1,22 @@
+using System.Text.Json;
+using ClaudeDo.Ui;
+
+namespace ClaudeDo.Ui.Tests;
+
+public class AppSettingsTests
+{
+    [Fact]
+    public void Language_defaults_to_empty()
+    {
+        Assert.Equal("", new AppSettings().Language);
+    }
+
+    [Fact]
+    public void Language_round_trips_through_json()
+    {
+        var json = JsonSerializer.Serialize(new AppSettings { Language = "de" });
+        var back = JsonSerializer.Deserialize<AppSettings>(json,
+            new JsonSerializerOptions { PropertyNameCaseInsensitive = true })!;
+        Assert.Equal("de", back.Language);
+    }
+}