feat(i18n): add CultureResolver for OS-culture mapping

src/ClaudeDo.Localization/CultureResolver.cs

@@ -0,0 +1,16 @@
+namespace ClaudeDo.Localization;
+
+public static class CultureResolver
+{
+    public static string Resolve(string cultureName, IReadOnlyCollection<string> available, string fallback = "en")
+    {
+        if (string.IsNullOrWhiteSpace(cultureName)) return fallback;
+
+        var exact = available.FirstOrDefault(c => string.Equals(c, cultureName, StringComparison.OrdinalIgnoreCase));
+        if (exact is not null) return exact;
+
+        var primary = cultureName.Split('-')[0];
+        var byPrimary = available.FirstOrDefault(c => string.Equals(c, primary, StringComparison.OrdinalIgnoreCase));
+        return byPrimary ?? fallback;
+    }
+}