c2a966a6ea
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