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);
+    }
+}

tests/ClaudeDo.Localization.Tests/LocalizerTests.cs

@@ -0,0 +1,58 @@
+using ClaudeDo.Localization;
+
+namespace ClaudeDo.Localization.Tests;
+
+public class LocalizerTests
+{
+    private static LocaleStore TwoLangStore()
+    {
+        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" }, "msg": { "count": "{0} items" } }""");
+        File.WriteAllText(Path.Combine(dir, "de.json"),
+            """{ "metadata": { "code": "de", "name": "Deutsch" }, "settings": { "save": "Speichern" } }""");
+        return LocaleStore.Load(dir);
+    }
+
+    [Fact]
+    public void Indexer_returns_active_language_value()
+    {
+        var loc = new Localizer(TwoLangStore(), "de", fallbackCode: "en");
+        Assert.Equal("Speichern", loc["settings.save"]);
+    }
+
+    [Fact]
+    public void Indexer_falls_back_to_english_then_key()
+    {
+        var loc = new Localizer(TwoLangStore(), "de", fallbackCode: "en");
+        Assert.Equal("{0} items", loc["msg.count"]);
+        Assert.Equal("nope.missing", loc["nope.missing"]);
+    }
+
+    [Fact]
+    public void Get_formats_arguments()
+    {
+        var loc = new Localizer(TwoLangStore(), "en", fallbackCode: "en");
+        Assert.Equal("3 items", loc.Get("msg.count", 3));
+    }
+
+    [Fact]
+    public void SetLanguage_raises_change_and_switches_values()
+    {
+        var loc = new Localizer(TwoLangStore(), "en", fallbackCode: "en");
+        var raised = 0;
+        loc.LanguageChanged += (_, _) => raised++;
+        loc.SetLanguage("de");
+        Assert.Equal("de", loc.CurrentCode);
+        Assert.Equal("Speichern", loc["settings.save"]);
+        Assert.Equal(1, raised);
+    }
+
+    [Fact]
+    public void AvailableLanguages_lists_code_and_name()
+    {
+        var loc = new Localizer(TwoLangStore(), "en", fallbackCode: "en");
+        Assert.Contains(loc.AvailableLanguages, l => l.Code == "de" && l.Name == "Deutsch");
+    }
+}