refactor: implement script tag approach for library inclusion

- Add script tags to demo-site HTML files for manual development
- Disable CLI inline script injection to prevent duplicate scripts
- Add library serving endpoints to servedev command
- Update build process to auto-copy library to demo-site
- Add CDN URL helpers for future production deployment
- Update .gitignore for generated demo-site files

Fixes .insertr-gate authentication for manual npm run serve workflow
while maintaining clean separation between CLI and manual setups.

demo-site/about.html

@@ -105,6 +105,7 @@
     </footer>
 
     <!-- Insertr JavaScript Library -->
+    <script type="text/javascript" src="insertr.js"></script>
 
 </body>
 </html>