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

lib/Txt/Txt.cpp

@@ -155,6 +155,21 @@ bool Txt::generateCoverBmp() const {
   return false;
 }
 
+bool Txt::clearCache() const {
+  if (!Storage.exists(cachePath.c_str())) {
+    LOG_DBG("TXT", "Cache does not exist, no action needed");
+    return true;
+  }
+
+  if (!Storage.removeDir(cachePath.c_str())) {
+    LOG_ERR("TXT", "Failed to clear cache");
+    return false;
+  }
+
+  LOG_DBG("TXT", "Cache cleared successfully");
+  return true;
+}
+
 bool Txt::readContent(uint8_t* buffer, size_t offset, size_t length) const {
   if (!loaded) {
     return false;

lib/Txt/Txt.h

@@ -22,6 +22,7 @@ class Txt {
   [[nodiscard]] size_t getFileSize() const { return fileSize; }
 
   void setupCacheDir() const;
+  bool clearCache() const;
 
   // Cover image support - looks for cover.bmp/jpg/jpeg/png in same folder as txt file
   [[nodiscard]] std::string getCoverBmpPath() const;