feat(i18n): add language dropdown to settings and persist selection

tests/ClaudeDo.Ui.Tests/LanguageSettingTests.cs

@@ -0,0 +1,35 @@
+using ClaudeDo.Localization;
+using ClaudeDo.Ui;
+using ClaudeDo.Ui.ViewModels.Modals.Settings;
+
+namespace ClaudeDo.Ui.Tests;
+
+public class LanguageSettingTests
+{
+    private static Localizer Make()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), "loc_" + Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(dir);
+        File.WriteAllText(Path.Combine(dir, "en.json"),
+            """{ "metadata": { "code": "en", "name": "English" }, "settings": { "save": "Save" } }""");
+        File.WriteAllText(Path.Combine(dir, "de.json"),
+            """{ "metadata": { "code": "de", "name": "Deutsch" }, "settings": { "save": "Speichern" } }""");
+        return new Localizer(LocaleStore.Load(dir), "en");
+    }
+
+    [Fact]
+    public void Selecting_language_switches_localizer_and_persists()
+    {
+        var loc = Make();
+        var settings = new AppSettings();
+        var saved = false;
+        var vm = new GeneralSettingsTabViewModel(loc, code => { settings.Language = code; saved = true; });
+
+        var de = vm.Languages.First(l => l.Code == "de");
+        vm.SelectedLanguage = de;
+
+        Assert.Equal("de", loc.CurrentCode);
+        Assert.True(saved);
+        Assert.Equal("de", settings.Language);
+    }
+}