refactor: unify book cache clearing for epub, txt, and xtc files (#1875)

src/util/BookCacheUtils.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <string>
+
+// Clears the reading cache for a book file if its extension is recognised
+// (EPUB, XTC, or TXT). Does nothing for other file types.
+void clearBookCache(const std::string& path);
+
+// Returns true if the directory name matches a book cache entry.
+bool isBookCacheDirectoryName(const char* name);