feat: refine letters UI

app/books/kb/[volume]/[slug]/page.tsx

@@ -35,7 +35,7 @@ export default async function KbChapterPage({
 
   return (
     <div className="min-h-screen bg-background">
-      <PageHeader sticky wide>
+      <PageHeader sticky>
         <Link
           href={`/books/kb/${vol}`}
           className="text-sm text-muted-foreground hover:text-foreground transition-colors"
@@ -50,7 +50,7 @@ export default async function KbChapterPage({
         </h1>
       </PageHeader>
 
-      <div className="w-full md:max-w-6xl mx-auto flex">
+      <div className="max-w-5xl mx-auto flex">
         {/* Left sidebar — chapter index */}
         <ChapterSidebar
           chapters={allChapters}