fix(sync): skip CRE->CFI conversion for image-based fixed formats

The CRE->CFI converter works by text search / character-offset mapping
across the EPUB spine. Image-based fixed content (fixed-layout comic
EPUBs, PDFs, comic archives) has no extractable text, so the conversion
can never succeed and only wastes time parsing content docs while
logging a failed percentage-precision result.

Guard the conversion in the KOReader progress handler: when the matched
media item's FormatGroup is fixed_layout or comic_archive, skip
ConvertCREToStandard entirely. The incoming xpointer is left as-is so
KOReader<->KOReader restore via GotoXPointer still works; the web reader
restores by page index (the canonical locator for these formats).

This covers fixed-layout comic EPUBs in particular: KOReader routes all
EPUBs through CREngine (has_pages == false), so they send a real
xpointer that passes IsCREXPointer and would otherwise trigger the
doomed text extraction. Reflowable EPUBs (format_group == reflowable)
still run the conversion exactly as before.

internal/handlers/koreader.go

@@ -457,6 +457,14 @@ func (h *KOReaderHandler) updateProgressForBook(c *echo.Context, deviceID pgtype
 			mediaItem, err := h.db.GetMediaItem(ctx, mediaItemID)
 			if err != nil {
 				log.Printf("Bookhoard: CRE→CFI failed to get media item: %v", err)
+			} else if mediaItem.FormatGroup == string(wsync.FormatGroupFixedLayout) ||
+				mediaItem.FormatGroup == string(wsync.FormatGroupComicArchive) {
+				// Image-based fixed content (fixed-layout comic EPUBs, PDF,
+				// comic archives) has no extractable text, so CRE→CFI conversion
+				// cannot succeed. The page index (page/total_pages) is the
+				// canonical locator. Keep the incoming xpointer for device-native
+				// restore; the web reader restores by page.
+				log.Printf("Bookhoard: CRE→CFI skipped for %s format", mediaItem.FormatGroup)
 			} else if h.libraryService == nil {
 				log.Printf("Bookhoard: CRE→CFI libraryService is nil, skipping conversion")
 			} else {