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/Steps/WriteConfigStep.cs

@@ -31,6 +31,7 @@ public sealed class WriteConfigStep : IInstallStep
             {
                 DbPath = Paths.Expand(ctx.UiDbPath),
                 SignalRUrl = ctx.SignalRUrl,
+                Language = ctx.Language,
             };
             uiCfg.Save();
             progress.Report("Written ui.config.json");