feat: add BookManager utility and RecentBooksStore::clear()

BookManager provides static functions for archive/unarchive/delete/
deleteCache/reindex operations on books, centralizing cache path
computation and file operations. Archive preserves directory structure
under /.archive/ and renames cache dirs to match new path hashes.

RecentBooksStore: :clear() added for bulk cache clearing use case.
Co-authored-by: Cursor <cursoragent@cursor.com>

src/RecentBooksStore.cpp

@@ -47,6 +47,11 @@ void RecentBooksStore::removeBook(const std::string& path) {
   }
 }
 
+void RecentBooksStore::clear() {
+  recentBooks.clear();
+  saveToFile();
+}
+
 void RecentBooksStore::updateBook(const std::string& path, const std::string& title, const std::string& author,
                                   const std::string& coverBmpPath) {
   auto it =