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;
+}

src/ClaudeDo.Localization/Localizer.cs

@@ -0,0 +1,55 @@
+namespace ClaudeDo.Localization;
+
+public sealed class Localizer : ILocalizer
+{
+    private readonly LocaleStore _store;
+    private readonly string _fallbackCode;
+    private LocaleFile? _active;
+    private LocaleFile? _fallback;
+
+    public Localizer(LocaleStore store, string code, string fallbackCode = "en")
+    {
+        _store = store;
+        _fallbackCode = fallbackCode;
+        _store.TryGet(fallbackCode, out _fallback);
+        SetLanguage(code);
+    }
+
+    public string CurrentCode { get; private set; } = "";
+
+    public IReadOnlyList<LanguageOption> AvailableLanguages =>
+        _store.Available.Select(f => new LanguageOption(f.Code, f.Name)).ToList();
+
+    public event EventHandler? LanguageChanged;
+
+    public string this[string key]
+    {
+        get
+        {
+            if (_active is not null && _active.Strings.TryGetValue(key, out var v)) return v;
+            if (_fallback is not null && _fallback.Strings.TryGetValue(key, out var fv)) return fv;
+            return key;
+        }
+    }
+
+    public string Get(string key, params object[] args)
+    {
+        var fmt = this[key];
+        return args.Length == 0 ? fmt : string.Format(fmt, args);
+    }
+
+    public void SetLanguage(string code)
+    {
+        if (_store.TryGet(code, out var f) && f is not null)
+        {
+            _active = f;
+            CurrentCode = f.Code;
+        }
+        else
+        {
+            _active = _fallback;
+            CurrentCode = _fallback?.Code ?? code;
+        }
+        LanguageChanged?.Invoke(this, EventArgs.Empty);
+    }
+}