Add paragraph index LUT for accurate KOReader position sync

Store per-page paragraph indices in section cache to enable precise
XPath-to-page and page-to-XPath mapping without reparsing XHTML.

Forward path (upload): generates XPath directly from paragraph LUT
instead of byte-offset estimation, eliminating drift in chapters
with non-uniform content density.

Reverse path (download): resolves incoming KOReader XPath p[N] to
the exact page via paragraph LUT lookup.

Paragraph counter counts all <p> elements including display:none
to match ChapterXPathIndexer and crengine's standard XPath counting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/activities/reader/EpubReaderActivity.cpp

@@ -395,9 +395,18 @@ void EpubReaderActivity::onReaderMenuConfirm(EpubReaderMenuActivity::MenuAction
       if (KOREADER_STORE.hasCredentials()) {
         const int currentPage = section ? section->currentPage : 0;
         const int totalPages = section ? section->pageCount : 0;
+        // Look up paragraph index from section cache for accurate XPath generation on upload
+        uint16_t paragraphIdx = 0;
+        bool hasParagraphIdx = false;
+        if (section) {
+          if (const auto pIdx = section->getParagraphIndexForPage(currentPage)) {
+            paragraphIdx = *pIdx;
+            hasParagraphIdx = true;
+          }
+        }
         startActivityForResult(
             std::make_unique<KOReaderSyncActivity>(renderer, mappedInput, epub, epub->getPath(), currentSpineIndex,
-                                                   currentPage, totalPages),
+                                                   currentPage, totalPages, paragraphIdx, hasParagraphIdx),
             [this](const ActivityResult& result) {
               if (!result.isCancelled) {
                 const auto& sync = std::get<SyncResult>(result.data);
@@ -405,6 +414,10 @@ void EpubReaderActivity::onReaderMenuConfirm(EpubReaderMenuActivity::MenuAction
                   RenderLock lock(*this);
                   currentSpineIndex = sync.spineIndex;
                   nextPageNumber = sync.page;
+                  if (sync.hasParagraphIndex) {
+                    pendingParagraphLookup = true;
+                    pendingParagraphIndex = sync.paragraphIndex;
+                  }
                   section.reset();
                 }
               }
@@ -623,6 +636,17 @@ void EpubReaderActivity::render(RenderLock&& lock) {
       pendingAnchor.clear();
     }
 
+    // Resolve pending KOReader sync paragraph index to accurate page via Section paragraph LUT
+    if (pendingParagraphLookup) {
+      if (const auto page = section->getPageForParagraphIndex(pendingParagraphIndex)) {
+        section->currentPage = *page;
+        LOG_DBG("ERS", "Resolved p[%u] to page %d (was %d)", pendingParagraphIndex, *page, nextPageNumber);
+      } else {
+        LOG_DBG("ERS", "Paragraph LUT not available, using estimated page %d", nextPageNumber);
+      }
+      pendingParagraphLookup = false;
+    }
+
     // handles changes in reader settings and reset to approximate position based on cached progress
     if (cachedChapterTotalPageCount > 0) {
       // only goes to relative position if spine index matches cached value