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.h

@@ -0,0 +1,72 @@
+#pragma once
+#include <Epub.h>
+
+#include <memory>
+#include <string>
+
+/**
+ * CrossPoint position representation.
+ */
+struct CrossPointPosition {
+  int spineIndex;  // Current spine item (chapter) index
+  int pageNumber;  // Current page within the spine item
+  int totalPages;  // Total pages in the current spine item
+};
+
+/**
+ * KOReader position representation.
+ */
+struct KOReaderPosition {
+  std::string xpath;  // XPath-like progress string
+  float percentage;   // Progress percentage (0.0 to 1.0)
+};
+
+/**
+ * Maps between CrossPoint and KOReader position formats.
+ *
+ * CrossPoint tracks position as (spineIndex, pageNumber).
+ * KOReader uses XPath-like strings + percentage.
+ *
+ * Since CrossPoint discards HTML structure during parsing, we generate
+ * synthetic XPath strings based on spine index, using percentage as the
+ * primary sync mechanism.
+ */
+class ProgressMapper {
+ public:
+  /**
+   * Convert CrossPoint position to KOReader format.
+   *
+   * @param epub The EPUB book
+   * @param pos CrossPoint position
+   * @return KOReader position
+   */
+  static KOReaderPosition toKOReader(const std::shared_ptr<Epub>& epub, const CrossPointPosition& pos);
+
+  /**
+   * Convert KOReader position to CrossPoint format.
+   *
+   * Note: The returned pageNumber may be approximate since different
+   * rendering settings produce different page counts.
+   *
+   * @param epub The EPUB book
+   * @param koPos KOReader position
+   * @param totalPagesInSpine Total pages in the target spine item (for page estimation)
+   * @return CrossPoint position
+   */
+  static CrossPointPosition toCrossPoint(const std::shared_ptr<Epub>& epub, const KOReaderPosition& koPos,
+                                         int totalPagesInSpine = 0);
+
+ private:
+  /**
+   * Generate XPath for KOReader compatibility.
+   * Format: /body/DocFragment[spineIndex+1]/body/p[estimatedParagraph]
+   * Paragraph is estimated based on page position within the chapter.
+   */
+  static std::string generateXPath(int spineIndex, int pageNumber, int totalPages);
+
+  /**
+   * Parse DocFragment index from XPath string.
+   * Returns -1 if not found.
+   */
+  static int parseDocFragmentIndex(const std::string& xpath);
+};