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/home/HomeActivity.h

@@ -41,6 +41,12 @@ class HomeActivity final : public Activity {
   void freeCoverBuffer();     // Free the stored cover buffer
 
  public:
+  // Free cover buffer from external activities (e.g., when entering reader to reclaim memory)
+  static void freeCoverBufferIfAllocated();
+  
+  // Preload cover buffer from external activities (e.g., MyLibraryActivity) for instant Home screen
+  // Returns true if cover was successfully preloaded or already cached
+  static bool preloadCoverBuffer();
   explicit HomeActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,
                         const std::function<void()>& onContinueReading, const std::function<void()>& onListsOpen,
                         const std::function<void()>& onMyLibraryOpen, const std::function<void()>& onSettingsOpen,