Refactor to modular architecture with extensible configuration system

- Split monolithic insertr.js (932 lines) into 6 focused modules
- Extract configuration system for extensible field types and validation
- Separate validation, form rendering, content management, and markdown processing
- Maintain same API surface while improving maintainability and testability
- Update demo pages to use modular system
- Remove legacy support for cleaner codebase

demo-site/index.html

@@ -87,6 +87,11 @@
     <!-- Insertr JavaScript Library -->
     <script src="https://cdn.jsdelivr.net/npm/marked@16.2.1/lib/marked.umd.js"></script>
     <script src="https://cdn.jsdelivr.net/npm/dompurify@3.0.5/dist/purify.min.js"></script>
+    <script src="insertr/config.js"></script>
+    <script src="insertr/validation.js"></script>
+    <script src="insertr/form-renderer.js"></script>
+    <script src="insertr/content-manager.js"></script>
+    <script src="insertr/markdown-processor.js"></script>
     <script src="insertr/insertr.js"></script>
 </body>
 </html>