feat(i18n): localize ViewModel-built strings via ambient Loc accessor

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Localization/Loc.cs

@@ -0,0 +1,43 @@
+using System;
+using System.Collections.Generic;
+using ClaudeDo.Localization;
+
+namespace ClaudeDo.Ui.Localization;
+
+/// Ambient access to the active localizer for code-built (ViewModel) strings.
+/// Set once at startup. Defaults to a key-echo localizer so unit tests that
+/// construct ViewModels without startup wiring do not crash.
+public static class Loc
+{
+    private static ILocalizer _current = new KeyEchoLocalizer();
+
+    public static ILocalizer Current
+    {
+        get => _current;
+        set
+        {
+            if (_current is not null) _current.LanguageChanged -= OnInnerChanged;
+            _current = value;
+            _current.LanguageChanged += OnInnerChanged;
+            OnInnerChanged(value, EventArgs.Empty);
+        }
+    }
+
+    public static event EventHandler? LanguageChanged;
+
+    private static void OnInnerChanged(object? sender, EventArgs e) =>
+        LanguageChanged?.Invoke(sender, e);
+
+    public static string T(string key) => Current[key];
+    public static string T(string key, params object[] args) => Current.Get(key, args);
+
+    private sealed class KeyEchoLocalizer : ILocalizer
+    {
+        public string this[string key] => key;
+        public string Get(string key, params object[] args) => key;
+        public string CurrentCode => "en";
+        public IReadOnlyList<LanguageOption> AvailableLanguages => Array.Empty<LanguageOption>();
+        public void SetLanguage(string code) { }
+        public event EventHandler? LanguageChanged { add { } remove { } }
+    }
+}