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/Pages/ServicePage/ServicePageViewModel.cs

@@ -1,5 +1,6 @@
 using System.Windows.Controls;
 using ClaudeDo.Installer.Core;
+using ClaudeDo.Installer.Localization;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
 using Microsoft.Win32;
@@ -11,7 +12,7 @@ public partial class ServicePageViewModel : ObservableObject, IInstallerPage
     private readonly InstallContext _context;
     private ServicePageView? _view;
 
-    public string Title => "Service";
+    public string Title => TrExtension.Localizer?["installer.service.title"] ?? "Service";
     public string Icon => "\uE912";
     public int Order => 2;
     public bool ShowInWizard => true;