Fix PDF position restore: timer after restore, add scroll-mode case

- Move auto-save timer setup to after position restore so an early
  visibilitychange can't overwrite the saved position with 0
- Add missing restore path for non-paginated PDF scroll mode

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

static/js/app.js

@@ -3157,7 +3157,7 @@ async function openBook(bookId) {
       progressInput.addEventListener('click', function () { this.select(); });
     }
 
-    // Restore scroll position
+    // Restore scroll position — must happen BEFORE auto-save timer is started
     try {
       const progressRes = await fetch('/books/');
       const allBooks = await progressRes.json();
@@ -3169,8 +3169,13 @@ async function openBook(bookId) {
         if (fraction > 0 && pdfTotalPages > 1)
           pdfCurrentPage = Math.max(1, Math.round(fraction * (pdfTotalPages - 1)) + 1);
         enterPdfPaginatedMode();
-      } else if (!isPdf) {
-        // Wait for all images so scrollHeight is final
+      } else if (isPdf) {
+        // PDF scroll mode
+        await new Promise(r => requestAnimationFrame(r));
+        if (fraction > 0)
+          contentEl.scrollTop = fraction * (contentEl.scrollHeight - contentEl.clientHeight);
+      } else {
+        // EPUB — wait for images so scrollHeight is final
         const imgs = Array.from(contentEl.querySelectorAll('img'));
         if (imgs.length) {
           await Promise.all(imgs.map(img =>
@@ -3206,6 +3211,7 @@ async function openBook(bookId) {
     });
 
     // Auto-save progress every 10s and on scroll (debounced 2s)
+    // Started AFTER restore so an early visibilitychange can't overwrite with position 0
     readerScrollSaveTimer = setInterval(saveReaderProgress, 10000);
     let _scrollDebounce = null;
     contentEl.addEventListener('scroll', () => {