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

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);
+    }
+}