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/extensions_ui/src/extensions_notice.dart

@@ -18,7 +18,7 @@ class ExtensionsNotice extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final tokens = ClideTheme.of(context).surface;
-    final i = ClideKernel.of(context).i18n;
+    final i = ClideSettings.i18n.of(context);
     return Column(
       crossAxisAlignment: CrossAxisAlignment.start,
       children: [