refactor(i18n): route consumer reads through ClideSettings.i18n (T-474)

Migrate widget i18n reads from ClideKernel.of(context).i18n to the unified
ClideSettings.i18n.of(context) facade (D-101), so settings/theme/fonts/i18n
share one widget-facing entry point. The facade delegates to the same I18n
service, so behaviour is unchanged. Infra plumbing (KernelServices /
ExtensionManager fields, the slot-host Listenable.merge) keeps its direct
kernel reference.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/theme_picker/src/appearance_control.dart

@@ -19,7 +19,7 @@ class AppearanceThemeControl extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final controller = ClideKernel.of(context).theme;
-    final i = ClideKernel.of(context).i18n;
+    final i = ClideSettings.i18n.of(context);
     return ListenableBuilder(
       listenable: controller,
       builder: (context, _) {

lib/builtin/theme_picker/src/settings_view.dart

@@ -45,9 +45,8 @@ class _SettingsViewState extends State<SettingsView> {
 
   @override
   Widget build(BuildContext context) {
-    final kernel = ClideKernel.of(context);
     final tokens = ClideTheme.of(context).surface;
-    final i = kernel.i18n;
+    final i = ClideSettings.i18n.of(context);
     final themes = baseThemes(widget.controller.available);
     final currentBase = baseThemeName(widget.controller.currentName);
     final title = i.string('modal.title', namespace: SettingsView.ns, placeholder: 'Settings');