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/menubar/src/about_dialog.dart

@@ -13,7 +13,7 @@ class AboutDialog extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     return Container(
       width: 480,
       padding: const EdgeInsets.all(20),

lib/builtin/menubar/src/file_actions.dart

@@ -110,7 +110,7 @@ class _OpenFolderDialogState extends State<OpenFolderDialog> {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     return Container(
       width: 420,
       padding: const EdgeInsets.all(20),
@@ -173,7 +173,7 @@ class NotARepoDialog extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     return Container(
       width: 420,
       padding: const EdgeInsets.all(20),

lib/builtin/menubar/src/menu_bar.dart

@@ -142,7 +142,7 @@ class _TopMenuButtonState extends State<_TopMenuButton> {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     final open = widget.controller.openIndex == widget.index;
     return ClideAnchoredOverlay(
       controller: _overlay,
@@ -166,7 +166,7 @@ class _TopMenuButtonState extends State<_TopMenuButton> {
                         node.keybinding!,
                         fontSize: clideFontSmall,
                         fontFamily: ClideSettings.fonts.monoOf(context),
-                        color: ClideTheme.of(ctx).surface.globalTextMuted,
+                        color: ClideSettings.theme.of(ctx).surface.globalTextMuted,
                       )
                     : null,
                 onSelect: () => unawaited(widget.kernel.commands.execute(node.commandId)),