feat: epub bookmarks (#1337)

Co-authored-by: vedi0boy <nate@origin8publishing.com>
Co-authored-by: Uri Tauber <uritaube@gmail.com>

src/BookmarkEntry.h

@@ -0,0 +1,14 @@
+#pragma once
+#include <cstdint>
+#include <string>
+
+// A single bookmark entry — a position in a book.
+struct BookmarkEntry {
+  std::string xpath;    // XPath-like progress string
+  std::string summary;  // First few words of a page to help identify it
+  float percentage;     // Progress percentage (0.0 to 1.0)
+
+  uint16_t computedSpineIndex = 0;        // Spine index at the time of bookmarking
+  uint16_t computedChapterPageCount = 0;  // Total page count of the chapter at the time of bookmarking
+  uint16_t computedChapterProgress = 0;   // Number of pages into the chapter at the time of bookmarking
+};