Adds KOReader Sync support (#232)

## Summary

- Adds KOReader progress sync integration, allowing CrossPoint to sync
reading positions with other
KOReader-compatible devices
- Stores credentials securely with XOR obfuscation
- Uses KOReader's partial MD5 document hashing for cross-device book
matching
  - Syncs position via percentage with estimated XPath for compatibility

# Features
- Settings: KOReader Username, Password, and Authenticate options
- Sync from chapters menu: "Sync Progress" option appears when
credentials are configured
- Bidirectional sync: Can apply remote progress or upload local progress

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

lib/KOReaderSync/ProgressMapper.cpp

@@ -0,0 +1,112 @@
+#include "ProgressMapper.h"
+
+#include <HardwareSerial.h>
+
+#include <cmath>
+
+KOReaderPosition ProgressMapper::toKOReader(const std::shared_ptr<Epub>& epub, const CrossPointPosition& pos) {
+  KOReaderPosition result;
+
+  // Calculate page progress within current spine item
+  float intraSpineProgress = 0.0f;
+  if (pos.totalPages > 0) {
+    intraSpineProgress = static_cast<float>(pos.pageNumber) / static_cast<float>(pos.totalPages);
+  }
+
+  // Calculate overall book progress (0.0-1.0)
+  result.percentage = epub->calculateProgress(pos.spineIndex, intraSpineProgress);
+
+  // Generate XPath with estimated paragraph position based on page
+  result.xpath = generateXPath(pos.spineIndex, pos.pageNumber, pos.totalPages);
+
+  // Get chapter info for logging
+  const int tocIndex = epub->getTocIndexForSpineIndex(pos.spineIndex);
+  const std::string chapterName = (tocIndex >= 0) ? epub->getTocItem(tocIndex).title : "unknown";
+
+  Serial.printf("[%lu] [ProgressMapper] CrossPoint -> KOReader: chapter='%s', page=%d/%d -> %.2f%% at %s\n", millis(),
+                chapterName.c_str(), pos.pageNumber, pos.totalPages, result.percentage * 100, result.xpath.c_str());
+
+  return result;
+}
+
+CrossPointPosition ProgressMapper::toCrossPoint(const std::shared_ptr<Epub>& epub, const KOReaderPosition& koPos,
+                                                int totalPagesInSpine) {
+  CrossPointPosition result;
+  result.spineIndex = 0;
+  result.pageNumber = 0;
+  result.totalPages = totalPagesInSpine;
+
+  const size_t bookSize = epub->getBookSize();
+  if (bookSize == 0) {
+    return result;
+  }
+
+  // First, try to get spine index from XPath (DocFragment)
+  int xpathSpineIndex = parseDocFragmentIndex(koPos.xpath);
+  if (xpathSpineIndex >= 0 && xpathSpineIndex < epub->getSpineItemsCount()) {
+    result.spineIndex = xpathSpineIndex;
+    // When we have XPath, go to page 0 of the spine - byte-based page calculation is unreliable
+    result.pageNumber = 0;
+  } else {
+    // Fall back to percentage-based lookup for both spine and page
+    const size_t targetBytes = static_cast<size_t>(bookSize * koPos.percentage);
+
+    // Find the spine item that contains this byte position
+    for (int i = 0; i < epub->getSpineItemsCount(); i++) {
+      const size_t cumulativeSize = epub->getCumulativeSpineItemSize(i);
+      if (cumulativeSize >= targetBytes) {
+        result.spineIndex = i;
+        break;
+      }
+    }
+
+    // Estimate page number within the spine item using percentage (only when no XPath)
+    if (totalPagesInSpine > 0 && result.spineIndex < epub->getSpineItemsCount()) {
+      const size_t prevCumSize = (result.spineIndex > 0) ? epub->getCumulativeSpineItemSize(result.spineIndex - 1) : 0;
+      const size_t currentCumSize = epub->getCumulativeSpineItemSize(result.spineIndex);
+      const size_t spineSize = currentCumSize - prevCumSize;
+
+      if (spineSize > 0) {
+        const size_t bytesIntoSpine = (targetBytes > prevCumSize) ? (targetBytes - prevCumSize) : 0;
+        const float intraSpineProgress = static_cast<float>(bytesIntoSpine) / static_cast<float>(spineSize);
+        const float clampedProgress = std::max(0.0f, std::min(1.0f, intraSpineProgress));
+        result.pageNumber = static_cast<int>(clampedProgress * totalPagesInSpine);
+        result.pageNumber = std::max(0, std::min(result.pageNumber, totalPagesInSpine - 1));
+      }
+    }
+  }
+
+  Serial.printf("[%lu] [ProgressMapper] KOReader -> CrossPoint: %.2f%% at %s -> spine=%d, page=%d\n", millis(),
+                koPos.percentage * 100, koPos.xpath.c_str(), result.spineIndex, result.pageNumber);
+
+  return result;
+}
+
+std::string ProgressMapper::generateXPath(int spineIndex, int pageNumber, int totalPages) {
+  // KOReader uses 1-based DocFragment indices
+  // Use a simple xpath pointing to the DocFragment - KOReader will use the percentage for fine positioning
+  // Avoid specifying paragraph numbers as they may not exist in the target document
+  return "/body/DocFragment[" + std::to_string(spineIndex + 1) + "]/body";
+}
+
+int ProgressMapper::parseDocFragmentIndex(const std::string& xpath) {
+  // Look for DocFragment[N] pattern
+  const size_t start = xpath.find("DocFragment[");
+  if (start == std::string::npos) {
+    return -1;
+  }
+
+  const size_t numStart = start + 12;  // Length of "DocFragment["
+  const size_t numEnd = xpath.find(']', numStart);
+  if (numEnd == std::string::npos) {
+    return -1;
+  }
+
+  try {
+    const int docFragmentIndex = std::stoi(xpath.substr(numStart, numEnd - numStart));
+    // KOReader uses 1-based indices, we use 0-based
+    return docFragmentIndex - 1;
+  } catch (...) {
+    return -1;
+  }
+}