refactor(theme): route consumer reads through ClideSettings.theme (T-473)

Migrate ClideTheme.of(context) reads across the widget, feature, and shell
layers to the unified ClideSettings.theme.of(context) facade (D-101), so
theme/i18n/fonts/settings share one widget-facing entry. The facade delegates
straight to ClideTheme, so behaviour is unchanged — goldens are unmoved.

The low-level theme provider keeps its direct ClideTheme.of: the facade is
built on it, and the two kernel sites (ClideTheme's own definition + the
panels drag-resize widget) stay direct to avoid a widgets→kernel import cycle.
Dead controller.dart/kernel.dart imports left by the sweep removed.

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

lib/builtin/search/src/search_panel_view.dart

@@ -66,7 +66,7 @@ class _SearchPanelViewState extends State<SearchPanelView> {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     return Column(
       crossAxisAlignment: CrossAxisAlignment.stretch,
       children: [
@@ -90,7 +90,7 @@ class _SearchPanelViewState extends State<SearchPanelView> {
   }
 
   Widget _findBody() {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     final c = _controller!;
     return ListenableBuilder(
       listenable: c,
@@ -443,7 +443,7 @@ class _MessageDialog extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     return _DialogFrame(
       tokens: tokens,
       children: [
@@ -468,7 +468,7 @@ class _ConfirmDialog extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     return _DialogFrame(
       tokens: tokens,
       children: [