fix(sync): treat page index as sole locator for fixed-layout pull path

The push path (koreader→server) was already updated to omit epubcfi
for paging documents, and SaveProgress derives percentage from
page/total_pages for fixed formats. However, the pull path
(server→koreader) still returned any stored epubcfi and attempted
CFI→XPointer conversion, and SaveProgress preserved stale CFI values
written by the web reader (which generates fake CFIs via
CFI.fake.fromIndex for every comic/PDF page).

These fake CFI strings lingered in the database and koreader's pull
path picked them up over progress.page, causing tonumber("epubcfi(...)")
→ nil → GotoPage(nil) → crash when the user confirmed the sync prompt.

Changes:
- GetMetadata (koreader.go): for fixed_layout/comic_archive formats,
  skip returning epubcfi and skip the CFI→CRE XPointer conversion.
  Reflowable documents are byte-for-byte unchanged.
- SaveProgress (progress.go): for fixed formats, explicitly clear
  epubcfi and character_offset on every save so stale values from
  prior web-reader sessions are cleaned up over time.

internal/handlers/koreader.go

@@ -622,11 +622,19 @@ func (h *KOReaderHandler) GetMetadata(c *echo.Context) error {
 		Percentage: progress.Percentage.Float64,
 	}
 
-	if progress.Epubcfi.Valid {
-		progressData.Epubcfi = &progress.Epubcfi.String
-	}
-	if progress.Epubcfi.Valid && wsync.IsStandardEPUBCFI(progress.Epubcfi.String) {
-		h.convertCFIToXPointer(c, mediaItem, progress, &progressData)
+	// CFI/xpointer are meaningless for image-based fixed-layout content; the
+	// page index is the canonical locator. Only return them for reflowable docs.
+	formatGroup := wsync.FormatGroup(mediaItem.FormatGroup)
+	isFixed := formatGroup == wsync.FormatGroupFixedLayout ||
+		formatGroup == wsync.FormatGroupComicArchive
+
+	if !isFixed {
+		if progress.Epubcfi.Valid {
+			progressData.Epubcfi = &progress.Epubcfi.String
+		}
+		if progress.Epubcfi.Valid && wsync.IsStandardEPUBCFI(progress.Epubcfi.String) {
+			h.convertCFIToXPointer(c, mediaItem, progress, &progressData)
+		}
 	}
 	if progress.Chapter.Valid {
 		progress := int(progress.Chapter.Int32)