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/BookManager.cpp

@@ -12,7 +12,7 @@ namespace {
 constexpr const char* LOG_TAG = "BM";
 
 // Supported book extensions
-const char* SUPPORTED_EXTENSIONS[] = {".epub", ".txt", ".xtc", ".xtch"};
+const char* SUPPORTED_EXTENSIONS[] = {".epub", ".txt"};
 constexpr size_t SUPPORTED_EXTENSIONS_COUNT = sizeof(SUPPORTED_EXTENSIONS) / sizeof(SUPPORTED_EXTENSIONS[0]);
 }  // namespace
 
@@ -45,8 +45,6 @@ std::string BookManager::getCachePrefix(const std::string& path) {
     return "epub_";
   } else if (ext == ".txt") {
     return "txt_";
-  } else if (ext == ".xtc" || ext == ".xtch") {
-    return "xtc_";
   }
   return "";
 }