feat(i18n): localize installer with language picker and config write-through

- Init Localizer at app startup (before self-update prompt) and assign to TrExtension.Localizer
- Register ILocalizer in DI; inject into WizardViewModel and SettingsViewModel
- WizardViewModel: SelectedLanguage ComboBox binding with OnSelectedLanguageChanged -> SetLanguage + InstallContext.Language
- WizardWindow.xaml: DockPanel wraps step chips + language ComboBox (right-aligned)
- Localize all installer XAML: WizardWindow, WelcomePage, PathsPage, ServicePage, UiSettingsPage, InstallPage, SettingsWindow, SelfUpdatePromptWindow
- Localize page Title properties and WizardViewModel.NextButtonText via TrExtension.Localizer
- Persist chosen Language in WriteConfigStep and SettingsViewModel.Save into ui.config.json
- Append installer section to en.json (nav, welcome, paths, service, uiSettings, install, settings, selfUpdate)

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

src/ClaudeDo.Installer/App.xaml.cs

@@ -1,7 +1,12 @@
+using System.Globalization;
+using System.IO;
+using System.Linq;
 using System.Net.Http;
 using System.Reflection;
 using System.Windows;
 using ClaudeDo.Installer.Core;
+using ClaudeDo.Installer.Localization;
+using ClaudeDo.Localization;
 using ClaudeDo.Releases;
 using ClaudeDo.Installer.Pages.InstallPage;
 using ClaudeDo.Installer.Pages.PathsPage;
@@ -22,6 +27,17 @@ public partial class App : Application
     {
         base.OnStartup(e);
 
+        // --- Initialize localizer as early as possible so all windows can use {loc:Tr} ---
+        var localesDir = Path.Combine(AppContext.BaseDirectory, "locales");
+        var localeStore = LocaleStore.Load(localesDir);
+        var existingSettings = InstallerAppSettings.Load();
+        var initialLang = !string.IsNullOrWhiteSpace(existingSettings.Language)
+            ? existingSettings.Language
+            : CultureResolver.Resolve(CultureInfo.CurrentUICulture.Name,
+                  localeStore.Available.Select(l => l.Code).ToArray(), "en");
+        var localizer = new Localizer(localeStore, initialLang);
+        TrExtension.Localizer = localizer;
+
         // --- Self-update pre-flight ---
         // Resolve current exe path. Assembly.Location may point to a .dll for apphost-based
         // .NET apps; swap to the .exe companion when that happens.
@@ -120,7 +136,7 @@ public partial class App : Application
 
         // --- Existing wizard start-up unchanged below this line ---
 
-        _services = BuildServices();
+        _services = BuildServices(localizer);
 
         var context = _services.GetRequiredService<InstallContext>();
         context.InstallerVersion = GetInstallerVersion();
@@ -183,9 +199,10 @@ public partial class App : Application
         return infoAttr?.InformationalVersion ?? "0.0.0";
     }
 
-    private static ServiceProvider BuildServices()
+    private static ServiceProvider BuildServices(ILocalizer localizer)
     {
         var sc = new ServiceCollection();
+        sc.AddSingleton(localizer);
 
         // Core
         sc.AddSingleton<InstallContext>();