feat: floating bookmarks

add floating bookmarks

frontend/src/utils/epubRendition.ts

@@ -55,6 +55,23 @@ export function isEpubCfi(location: string | number): boolean {
   return typeof location === "string" && location.startsWith("epubcfi(");
 }
 
+/** Book progress 0–100 from an EPUB CFI (requires locations.generate). */
+export function percentageFromCfi(rendition: EpubRendition, cfi: string): number | null {
+  if (!rendition.book?.locations || !isEpubCfi(cfi)) return null;
+  try {
+    const pct = rendition.book.locations.percentageFromCfi(cfi);
+    if (pct === null || pct === undefined) return null;
+    return Math.min(100, Math.max(0, Math.round(pct * 100)));
+  } catch {
+    return null;
+  }
+}
+
+export function spineLength(rendition: EpubRendition): number {
+  const items = rendition.book?.spine?.spineItems;
+  return items?.length ?? 0;
+}
+
 /** Map nav/TOC href to a spine href epub.js can display. */
 export function resolveSpineHref(book: EpubBookForNav, href: string): string {
   const hashIndex = href.indexOf("#");