Fixed BookSelector search-to-breadcrumb gap - removed space-y-12 class

- **Space-y-12 removed**: Eliminated automatic 48px gaps between container children
- **Manual spacing preserved**: mb-4 on search and breadcrumbs now works as intended
- **Consistent with ChapterSelector**: Both components now have identical visual spacing
- **Tight, uniform gap**: 16px spacing between search bar and breadcrumbs everywhere

BookSelector and ChapterSelector now have perfectly matched visual spacing!

frontend/src/components/BookSelector.tsx

@@ -165,7 +165,7 @@ const BookSelector: React.FC<BookSelectorProps> = ({ books, onBookSelect, format
   );
 
   return (
-    <div className="space-y-12">
+    <div>
       {/* Search Bar */}
       <div className="flex justify-center mb-4">
         <div className="w-full max-w-md relative">