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

lib/Epub/Epub.cpp

@@ -251,8 +251,8 @@ bool Epub::parseCssFiles() {
     SdMan.remove(tmpCssPath.c_str());
   }
 
-  Serial.printf("[%lu] [EBP] Loaded %zu CSS style rules from %zu files\n", millis(), cssParser->ruleCount(),
-                cssFiles.size());
+  Serial.printf("[%lu] [EBP] Loaded %zu CSS style rules from %zu files (~%zu bytes)\n", millis(), cssParser->ruleCount(),
+                cssFiles.size(), cssParser->estimateMemoryUsage());
   return true;
 }
 

lib/Epub/Epub/css/CssParser.h

@@ -71,6 +71,25 @@ class CssParser {
    */
   [[nodiscard]] size_t ruleCount() const { return rulesBySelector_.size(); }
 
+  /**
+   * Estimate memory usage of loaded rules (for debugging)
+   * Returns approximate bytes used by selector strings and style data
+   */
+  [[nodiscard]] size_t estimateMemoryUsage() const {
+    size_t bytes = 0;
+    // unordered_map overhead: roughly 8 bytes per bucket + per-entry overhead
+    bytes += rulesBySelector_.bucket_count() * sizeof(void*);
+    for (const auto& entry : rulesBySelector_) {
+      // String storage: capacity + SSO overhead (~24 bytes) + actual chars
+      bytes += sizeof(std::string) + entry.first.capacity();
+      // CssStyle is ~16 bytes
+      bytes += sizeof(CssStyle);
+      // Per-entry node overhead in unordered_map (~24-32 bytes)
+      bytes += 32;
+    }
+    return bytes;
+  }
+
   /**
    * Clear all loaded rules
    */