feat(i18n): add Localizer with fallback chain and change event

src/ClaudeDo.Localization/ILocalizer.cs

@@ -0,0 +1,13 @@
+namespace ClaudeDo.Localization;
+
+public readonly record struct LanguageOption(string Code, string Name);
+
+public interface ILocalizer
+{
+    string this[string key] { get; }
+    string Get(string key, params object[] args);
+    string CurrentCode { get; }
+    IReadOnlyList<LanguageOption> AvailableLanguages { get; }
+    void SetLanguage(string code);
+    event EventHandler? LanguageChanged;
+}