refactor: Memory optimization and XTC format removal

Memory optimization:
- Add LOG_STACK_WATERMARK macro for task stack monitoring
- Add freeCoverBufferIfAllocated() and preloadCoverBuffer() for memory management
- Improve cover buffer reuse to reduce heap fragmentation
- Add grayscale buffer cleanup safety in GfxRenderer
- Make grayscale rendering conditional on successful buffer allocation
- Add detailed heap fragmentation logging with ESP-IDF API
- Add CSS parser memory usage estimation

XTC format removal:
- Remove entire lib/Xtc library (XTC parser and types)
- Remove XtcReaderActivity and XtcReaderChapterSelectionActivity
- Remove XTC file handling from HomeActivity, SleepActivity, ReaderActivity
- Remove .xtc/.xtch from supported extensions in BookManager
- Remove XTC cache prefix from Md5Utils
- Update web server and file browser to exclude XTC format
- Clear in-memory caches when disk cache is cleared

src/activities/reader/ReaderActivity.h

@@ -5,7 +5,6 @@
 #include "activities/home/MyLibraryActivity.h"
 
 class Epub;
-class Xtc;
 class Txt;
 
 class ReaderActivity final : public ActivityWithSubactivity {
@@ -15,15 +14,12 @@ class ReaderActivity final : public ActivityWithSubactivity {
   const std::function<void()> onGoBack;
   const std::function<void(const std::string&, MyLibraryActivity::Tab)> onGoToLibrary;
   static std::unique_ptr<Epub> loadEpub(const std::string& path);
-  static std::unique_ptr<Xtc> loadXtc(const std::string& path);
   static std::unique_ptr<Txt> loadTxt(const std::string& path);
-  static bool isXtcFile(const std::string& path);
   static bool isTxtFile(const std::string& path);
 
   static std::string extractFolderPath(const std::string& filePath);
   void goToLibrary(const std::string& fromBookPath = "");
   void onGoToEpubReader(std::unique_ptr<Epub> epub);
-  void onGoToXtcReader(std::unique_ptr<Xtc> xtc);
   void onGoToTxtReader(std::unique_ptr<Txt> txt);
 
  public: