Add a setting for document matching method

lib/KOReaderSync/KOReaderDocumentId.h

@@ -17,13 +17,22 @@
 class KOReaderDocumentId {
  public:
   /**
-   * Calculate the KOReader document hash for a file.
+   * Calculate the KOReader document hash for a file (binary/content-based).
    *
    * @param filePath Path to the file (typically an EPUB)
    * @return 32-character lowercase hex string, or empty string on failure
    */
   static std::string calculate(const std::string& filePath);
 
+  /**
+   * Calculate document hash from filename only (filename-based sync mode).
+   * This is simpler and works when files have the same name across devices.
+   *
+   * @param filePath Path to the file (only the filename portion is used)
+   * @return 32-character lowercase hex MD5 of the filename
+   */
+  static std::string calculateFromFilename(const std::string& filePath);
+
  private:
   // Size of each chunk to read at each offset
   static constexpr size_t CHUNK_SIZE = 1024;