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/EpubReaderActivity.cpp

@@ -190,6 +190,8 @@ void EpubReaderActivity::onExit() {
   // Wait until not rendering to delete task to avoid killing mid-instruction to EPD
   xSemaphoreTake(renderingMutex, portMAX_DELAY);
   if (displayTaskHandle) {
+    // Log stack high-water mark before deleting task (stack size: 8192 bytes)
+    LOG_STACK_WATERMARK("EpubReaderActivity", displayTaskHandle);
     vTaskDelete(displayTaskHandle);
     displayTaskHandle = nullptr;
     vTaskDelay(10 / portTICK_PERIOD_MS);  // Let idle task free stack
@@ -639,30 +641,33 @@ void EpubReaderActivity::renderContents(std::unique_ptr<Page> page, const int or
     pagesUntilFullRefresh--;
   }
 
-  // Save bw buffer to reset buffer state after grayscale data sync
-  renderer.storeBwBuffer();
-
-  // grayscale rendering
+  // grayscale rendering requires storing the BW buffer first
+  // If we can't allocate memory for the backup, skip grayscale to avoid artifacts
   // TODO: Only do this if font supports it
   if (SETTINGS.textAntiAliasing) {
-    renderer.clearScreen(0x00);
-    renderer.setRenderMode(GfxRenderer::GRAYSCALE_LSB);
-    page->render(renderer, SETTINGS.getReaderFontId(), orientedMarginLeft, orientedMarginTop);
-    renderer.copyGrayscaleLsbBuffers();
+    // Try to save BW buffer - if this fails, skip grayscale rendering entirely
+    const bool bwBufferStored = renderer.storeBwBuffer();
 
-    // Render and copy to MSB buffer
-    renderer.clearScreen(0x00);
-    renderer.setRenderMode(GfxRenderer::GRAYSCALE_MSB);
-    page->render(renderer, SETTINGS.getReaderFontId(), orientedMarginLeft, orientedMarginTop);
-    renderer.copyGrayscaleMsbBuffers();
+    if (bwBufferStored) {
+      renderer.clearScreen(0x00);
+      renderer.setRenderMode(GfxRenderer::GRAYSCALE_LSB);
+      page->render(renderer, SETTINGS.getReaderFontId(), orientedMarginLeft, orientedMarginTop);
+      renderer.copyGrayscaleLsbBuffers();
 
-    // display grayscale part
-    renderer.displayGrayBuffer();
-    renderer.setRenderMode(GfxRenderer::BW);
+      // Render and copy to MSB buffer
+      renderer.clearScreen(0x00);
+      renderer.setRenderMode(GfxRenderer::GRAYSCALE_MSB);
+      page->render(renderer, SETTINGS.getReaderFontId(), orientedMarginLeft, orientedMarginTop);
+      renderer.copyGrayscaleMsbBuffers();
+
+      // display grayscale part
+      renderer.displayGrayBuffer();
+      renderer.setRenderMode(GfxRenderer::BW);
+
+      // restore the bw data
+      renderer.restoreBwBuffer();
+    }
   }
-
-  // restore the bw data
-  renderer.restoreBwBuffer();
 }
 
 void EpubReaderActivity::renderStatusBar(const int orientedMarginRight, const int orientedMarginBottom,