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/Views/SettingsViewModel.cs

@@ -1,5 +1,6 @@
 using System.Windows;
 using ClaudeDo.Installer.Core;
+using ClaudeDo.Localization;
 using ClaudeDo.Releases;
 using ClaudeDo.Installer.Steps;
 using CommunityToolkit.Mvvm.ComponentModel;
@@ -10,6 +11,7 @@ namespace ClaudeDo.Installer.Views;
 public partial class SettingsViewModel : ObservableObject
 {
     private readonly InstallContext _context;
+    private readonly ILocalizer _localizer;
     private readonly IReleaseClient _releases;
     private readonly StopWorkerStep _stopService;
     private readonly StartWorkerStep _startService;
@@ -37,6 +39,7 @@ public partial class SettingsViewModel : ObservableObject
     public SettingsViewModel(
         PageResolver resolver,
         InstallContext context,
+        ILocalizer localizer,
         IReleaseClient releases,
         StopWorkerStep stopService,
         StartWorkerStep startService,
@@ -46,6 +49,7 @@ public partial class SettingsViewModel : ObservableObject
     {
         Pages = resolver.SettingsPages;
         _context = context;
+        _localizer = localizer;
         _releases = releases;
         _stopService = stopService;
         _startService = startService;
@@ -104,6 +108,7 @@ public partial class SettingsViewModel : ObservableObject
         {
             DbPath = _context.UiDbPath,
             SignalRUrl = _context.SignalRUrl,
+            Language = _localizer.CurrentCode,
         };
         uiCfg.Save();