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/theme_picker/src/appearance_control.dart

@@ -63,7 +63,7 @@ class _ThemeChip extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     return Semantics(
       button: true,
       selected: selected,
@@ -95,7 +95,7 @@ class _HcToggle extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     return Semantics(
       checked: checked,
       label: label,

lib/builtin/theme_picker/src/settings_view.dart

@@ -45,7 +45,7 @@ class _SettingsViewState extends State<SettingsView> {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     final i = ClideSettings.i18n.of(context);
     final themes = baseThemes(widget.controller.available);
     final currentBase = baseThemeName(widget.controller.currentName);
@@ -140,7 +140,7 @@ class _HighContrastToggleState extends State<_HighContrastToggle> {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     return Semantics(
       checked: widget.checked,
       label: widget.label,
@@ -200,7 +200,7 @@ class _ThemeRow extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     final bg = selected ? tokens.listItemSelectedBackground : (hovered ? tokens.listItemHoverBackground : tokens.listItemBackground);
     final fg = selected ? tokens.listItemSelectedForeground : tokens.listItemForeground;
     return Semantics(

lib/builtin/theme_picker/src/theme_status_item.dart

@@ -32,7 +32,7 @@ class _ThemeSwitcherStatusItemState extends State<ThemeSwitcherStatusItem> {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     // Listen to both the theme (label changes) and the overlay (open state).
     return ListenableBuilder(
       listenable: Listenable.merge([_controller, _overlay]),