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

src/ClaudeDo.Ui/Localization/TrExtension.cs

@@ -0,0 +1,25 @@
+using Avalonia.Data;
+using Avalonia.Markup.Xaml;
+using ClaudeDo.Localization;
+
+namespace ClaudeDo.Ui.Localization;
+
+public sealed class TrExtension : MarkupExtension
+{
+    public TrExtension() { }
+    public TrExtension(string key) => Key = key;
+
+    public string Key { get; set; } = "";
+
+    public static ILocalizer? Localizer { get; set; }
+
+    public override object ProvideValue(IServiceProvider serviceProvider)
+    {
+        var loc = Localizer ?? throw new InvalidOperationException("TrExtension.Localizer not initialized");
+        return new Binding(nameof(LocalizedString.Value))
+        {
+            Source = new LocalizedString(loc, Key),
+            Mode = BindingMode.OneWay
+        };
+    }
+}