fix(website): apply ThemeProvider to Playbook article layout so dark mode works

LearningArticleLayout replaces PageShell for Playbook articles but never
wrapped itself in a ThemeProvider, so its useTheme() call always fell back
to Primer's light default regardless of the site's actual theme preference.
Split the component into a thin ThemeProvider wrapper plus the existing
implementation (now LearningArticleLayoutBody), matching the pattern already
used by PageShell.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 80686fef-efe3-4cdd-8cd6-bfa61a5d0af6

website/src/components/brand/LearningArticleLayout.tsx

@@ -17,6 +17,7 @@ import {
   Heading,
   Section,
   Text,
+  ThemeProvider,
   useTheme,
 } from "@primer/react-brand";
 
@@ -144,7 +145,15 @@ export type LearningArticleLayoutProps = {
   children: React.ReactNode;
 };
 
-export function LearningArticleLayout({
+export function LearningArticleLayout(props: LearningArticleLayoutProps) {
+  return (
+    <ThemeProvider colorMode="auto">
+      <LearningArticleLayoutBody {...props} />
+    </ThemeProvider>
+  );
+}
+
+function LearningArticleLayoutBody({
   pageHref,
   currentPage,
   breadcrumbLabel,