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/Epub/Epub.cpp

@@ -609,14 +609,15 @@ int Epub::getSpineIndexForTextReference() const {
   return 0;
 }
 
-// Calculate progress in book
-uint8_t Epub::calculateProgress(const int currentSpineIndex, const float currentSpineRead) const {
+// Calculate progress in book (returns 0.0-1.0)
+float Epub::calculateProgress(const int currentSpineIndex, const float currentSpineRead) const {
   const size_t bookSize = getBookSize();
   if (bookSize == 0) {
-    return 0;
+    return 0.0f;
   }
   const size_t prevChapterSize = (currentSpineIndex >= 1) ? getCumulativeSpineItemSize(currentSpineIndex - 1) : 0;
   const size_t curChapterSize = getCumulativeSpineItemSize(currentSpineIndex) - prevChapterSize;
-  const size_t sectionProgSize = currentSpineRead * curChapterSize;
-  return round(static_cast<float>(prevChapterSize + sectionProgSize) / bookSize * 100.0);
+  const float sectionProgSize = currentSpineRead * static_cast<float>(curChapterSize);
+  const float totalProgress = static_cast<float>(prevChapterSize) + sectionProgSize;
+  return totalProgress / static_cast<float>(bookSize);
 }