Fix: Wrap starlightRoute access in try-catch for prerender safety

Handle cases where starlightRoute is not available during prerender of non-Starlight pages (e.g., 404 page).

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: isvictorious

src/starlight-route-middleware.ts

@@ -10,8 +10,12 @@ export const onRequest = defineRouteMiddleware(async (context, next) => {
   // so the sidebar has its final entries before we flatten single-child groups.
   await next();
 
-  const route = context.locals.starlightRoute;
-  if (route?.sidebar?.length) {
-    route.sidebar = flattenSidebarSingleChildGroups(route.sidebar);
+  try {
+    const route = context.locals.starlightRoute;
+    if (route?.sidebar?.length) {
+      route.sidebar = flattenSidebarSingleChildGroups(route.sidebar);
+    }
+  } catch {
+    // starlightRoute may not be available during prerender of non-Starlight pages (e.g., 404)
   }
 });