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/files/src/file_tree_view.dart

@@ -202,7 +202,7 @@ class _DirRow extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final expanded = controller.isExpanded(path);
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     final selected = path == selectedPath;
     return Semantics(
       button: true,
@@ -263,7 +263,7 @@ class _Row extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     final leftPadding = 8.0 + (depth * 14.0);
     return ClideTappable(
       onTap: onTap,
@@ -301,7 +301,7 @@ class _FilteredFileRow extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final tokens = ClideTheme.of(context).surface;
+    final tokens = ClideSettings.theme.of(context).surface;
     return ClideTappable(
       onTap: () => openWorkspaceFile(ClideKernel.of(context), entry.path),
       builder: (context, hovered, _) => Container(