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/boot_sleep/SleepActivity.cpp

@@ -4,7 +4,6 @@
 #include <GfxRenderer.h>
 #include <SDCardManager.h>
 #include <Txt.h>
-#include <Xtc.h>
 
 #include <cmath>
 
@@ -323,23 +322,8 @@ void SleepActivity::renderCoverSleepScreen() const {
 
   std::string coverBmpPath;
 
-  // Check if the current book is XTC, TXT, or EPUB
-  if (StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".xtc") ||
-      StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".xtch")) {
-    // Handle XTC file
-    Xtc lastXtc(APP_STATE.openEpubPath, "/.crosspoint");
-    if (!lastXtc.load()) {
-      Serial.println("[SLP] Failed to load last XTC");
-      return renderDefaultSleepScreen();
-    }
-
-    if (!lastXtc.generateCoverBmp()) {
-      Serial.println("[SLP] Failed to generate XTC cover bmp");
-      return renderDefaultSleepScreen();
-    }
-
-    coverBmpPath = lastXtc.getCoverBmpPath();
-  } else if (StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".txt")) {
+  // Check if the current book is TXT or EPUB
+  if (StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".txt")) {
     // Handle TXT file - looks for cover image in the same folder
     Txt lastTxt(APP_STATE.openEpubPath, "/.crosspoint");
     if (!lastTxt.load()) {
@@ -517,7 +501,7 @@ std::string SleepActivity::getCoverBmpPath(const std::string& cacheDir, const st
     return cropped ? (cacheDir + "/cover_crop.bmp") : (cacheDir + "/cover_fit.bmp");
   }
 
-  // XTC and TXT use a single cover.bmp
+  // TXT uses a single cover.bmp
   return cacheDir + "/cover.bmp";
 }