fix(progress): use page index for fixed-layout pull sync

The pull path (_doGetProgress + syncToProgress) still used the
reflowable-oriented nav_target selection that prefers xpointer/cfi
over page. For has_pages documents, if the server returned an epubcfi
(e.g., a fake CFI from the web reader's comic renderer), nav_target
became a CFI string, tonumber() returned nil, and GotoPage(nil)
crashed koreader when the user confirmed the sync prompt. After the
crash the old position was retained because GotoPage never completed.

This makes the pull path symmetric with the push path (ef129fd, which
omits epubcfi for paging documents):

- _doGetProgress: for has_pages, select nav_target from progress.page
  and show the actual page number in the sync prompt instead of
  computing it from server_percentage × total_pages (which could
  display the wrong page due to rounding differences between foliate
  and koreader page math).
- syncToProgress: add a nil guard so a missing page number skips
  gracefully instead of crashing.

main.lua

@@ -870,7 +870,10 @@ end
 function Bookhoard:syncToProgress(progress, percentage)
     logger.dbg("Bookhoard: sync to progress", progress, percentage)
     if self.ui.document.info.has_pages then
-        self.ui:handleEvent(Event:new("GotoPage", tonumber(progress)))
+        local page = tonumber(progress)
+        if page then
+            self.ui:handleEvent(Event:new("GotoPage", page))
+        end
     elseif progress and progress:match("^/body/") then
         self.ui:handleEvent(Event:new("GotoXPointer", progress))
     elseif percentage then
@@ -1052,17 +1055,21 @@ function Bookhoard:_doGetProgress(interactive)
 
         local self_older = server_percentage > percentage
 
-        local nav_target = progress.koreader_xpointer or progress.epubcfi or progress.page
-
+        local nav_target
         local sync_text
-        if not self.ui.document.info.has_pages then
+        if self.ui.document.info.has_pages then
+            -- Fixed-layout: the page index is the canonical locator. CFI/xpointer
+            -- are meaningless for image-based content, so use progress.page directly.
+            local total = progress.total_pages or self.ui.document:getPageCount()
+            local target_page = progress.page
+                or math.min(Math.round(server_percentage * total), total)
+            nav_target = progress.page
+            sync_text = T(_("Sync to page %1 of %2 from server?"), target_page, total)
+        else
+            nav_target = progress.koreader_xpointer or progress.epubcfi or progress.page
             local total = self.ui.document:getPageCount()
             local target_page = math.min(Math.round(server_percentage * total), total)
             sync_text = T(_("Sync to page %1 of %2 from server?"), target_page, total)
-        else
-            sync_text = T(_("Sync to page %1 of %2 from server?"),
-                Math.round(server_percentage * (progress.total_pages or 1)),
-                progress.total_pages or "?")
         end
 
         if self_older then