feat: Add kosync user registration and switch to crosspoint-sync server (#2587)

Implements createUser() endpoint to allow account creation via the
KOSync protocol. Changes default sync server from sync.koreader.rocks to
sync.crosspointreader.com with migration logic to preserve existing
users' server settings. Extends sync protocol to include position data
(spine index, page numbers, xpath) that crosspoint-sync supports while
remaining compatible with standard kosync servers.

lib/KOReaderSync/ProgressMapper.h

@@ -3,8 +3,11 @@
 #include <GfxRenderer.h>
 
 #include <memory>
+#include <optional>
 #include <string>
 
+#include "KOReaderSyncClient.h"
+
 /**
  * CrossPoint position representation.
  */
@@ -65,6 +68,20 @@ class ProgressMapper {
                                          GfxRenderer& renderer, int currentSpineIndex = -1,
                                          int totalPagesInCurrentSpine = 0, int fallbackTotalPages = 0);
 
+  /**
+   * Convert a rich CrossPoint position (downloaded from a crosspoint-sync
+   * server) directly to a CrossPoint position, without XPath approximation.
+   * When the local layout matches the uploader's (same spine page count) the
+   * page transfers losslessly; otherwise the paragraph LUT or the intra-spine
+   * page fraction is used.
+   *
+   * @return The position, or std::nullopt when the rich position cannot be
+   *         applied (spine out of range, no section cache) and the caller
+   *         should fall back to toCrossPoint().
+   */
+  static std::optional<CrossPointPosition> fromRichPosition(const std::shared_ptr<Epub>& epub,
+                                                            const KOReaderRichPosition& rich, GfxRenderer& renderer);
+
  private:
   /**
    * Generate a fallback XPath by streaming the spine item's XHTML and resolving