fix(reader): optimize rendering performance, prevent browser freeze on large technical books

- extract_outline_entries: filter PDF bookmarks by depth and major structural headings, avoiding 550 micro-snippets
- renderContinuousBook: load current chapter and seamlessly append next chapters on scroll (virtual continuous) instead of dumping all chapters into DOM synchronously
- renderInteractiveSpans: use native HTML template fragment instead of new DOMParser per paragraph
- event delegation: handle word clicks on readerContentBody instead of adding hundreds of thousands of individual DOM event listeners
- consolidate existing CCNA book in DB into 68 clean chapters

app/static/index.html

@@ -1075,7 +1075,7 @@
     <!-- Audio Element for native streaming -->
     <audio id="global-audio-player" preload="none"></audio>
 
-    <script src="/static/js/app.js?v=3.9"></script>
+    <script src="/static/js/app.js?v=4.0"></script>
 </body>
 </html>