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.

.gitignore

@@ -70,4 +70,7 @@ insertr-cli/build-errors.log
 *.sqlite3
 
 # Local development files
-.local/
+.local/
+
+# Demo site generated files
+demo-site/insertr.js