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

tests/ClaudeDo.Localization.Tests/CultureResolverTests.cs

@@ -0,0 +1,19 @@
+using ClaudeDo.Localization;
+
+namespace ClaudeDo.Localization.Tests;
+
+public class CultureResolverTests
+{
+    private static readonly string[] Codes = { "en", "de" };
+
+    [Theory]
+    [InlineData("de-DE", "de")]
+    [InlineData("de", "de")]
+    [InlineData("en-US", "en")]
+    [InlineData("fr-FR", "en")]
+    [InlineData("", "en")]
+    public void Resolve_maps_culture_to_available_code(string culture, string expected)
+    {
+        Assert.Equal(expected, CultureResolver.Resolve(culture, Codes, fallback: "en"));
+    }
+}