fix(reader): stop sending epubcfi for fixed-layout documents

The web reader's saveProgress always sent epubcfi: cfi || "", even
for fixed-layout comics/PDFs. Foliate's comic renderer generates a
fake CFI (epubcfi(/6/{n})) for every page via CFI.fake.fromIndex,
which the server stored as a valid locator. These fake CFIs are
meaningless — the page index is the canonical locator for image-based
content — and they caused koreader to crash on pull (see prior commit).

Only set epubcfi in the request body for reflowable documents.

web/src/reader/reader.ts

@@ -562,9 +562,11 @@ document.addEventListener("alpine:init", () => {
           ? this.book?.sections?.filter((s: any) => s.linear !== "no")
               ?.length ?? 0
           : location?.total ?? 0,
-        epubcfi: cfi || "",
         reading_mode: this.readingMode || undefined,
       };
+      if (!this.isFixedLayout) {
+        body.epubcfi = cfi || "";
+      }
 
       const tocItem = this.lastRelocateDetail?.tocItem;
       if (tocItem) {