test(ui): restore the language settings tests dropped in the previous commit

tests/ClaudeDo.Ui.Tests/AppSettingsTests.cs

@@ -9,6 +9,23 @@ public class AppSettingsTests
     private static string TempConfigPath() =>
         Path.Combine(Path.GetTempPath(), $"claudedo-uicfg-{Guid.NewGuid():N}.json");
 
+    [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);
+    }
+
     [Fact]
     public void DiffPreferences_DefaultToUnifiedAndNoWrap()
     {