feat(i18n): add Avalonia loc:Tr markup extension and LocalizedString

tests/ClaudeDo.Ui.Tests/LocalizedStringTests.cs

@@ -0,0 +1,33 @@
+using ClaudeDo.Localization;
+using ClaudeDo.Ui.Localization;
+
+namespace ClaudeDo.Ui.Tests;
+
+public class LocalizedStringTests
+{
+    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 Value_tracks_language_change()
+    {
+        var loc = Make();
+        var ls = new LocalizedString(loc, "settings.save");
+        Assert.Equal("Save", ls.Value);
+
+        string? changed = null;
+        ls.PropertyChanged += (_, e) => { if (e.PropertyName == nameof(LocalizedString.Value)) changed = ls.Value; };
+
+        loc.SetLanguage("de");
+        Assert.Equal("Speichern", ls.Value);
+        Assert.Equal("Speichern", changed);
+    }
+}