feat: implement flexible editor gate system

- Replace automatic auth controls with developer-placed .insertr-gate elements
- Add OAuth-ready authentication flow with mock implementation
- Support any HTML element as gate with custom styling
- Implement proper gate restoration after authentication
- Move auth controls to bottom-right corner for better UX
- Add editor gates to demo pages (footer link and styled button)
- Maintain gates visible by default with hideGatesAfterAuth option
- Prevent duplicate authentication attempts with loading states

This enables small business owners to access editor via discrete
footer links or custom-styled elements placed anywhere by developers.

demo-site/index.html

@@ -76,7 +76,7 @@
     <footer class="footer">
         <div class="container">
             <p class="insertr">&copy; 2024 Acme Consulting Services. All rights reserved.</p>
-            <p class="insertr">📧 info@acmeconsulting.com | 📞 (555) 123-4567</p>
+            <p class="insertr">📧 info@acmeconsulting.com | 📞 (555) 123-4567 | <a href="#" class="insertr-gate">Editor</a></p>
         </div>
     </footer>