Refactor: Replace sidebar with horizontal navigation bar

- Remove sidebar and toggle functionality
- Add Bootstrap navbar with dropdown menus
- Move navigation to top between header and content
- Update menu rendering for Bootstrap dropdowns
- Clean up unused files (header.mustache, sidebar.mustache, sidebar.js)
- Add guide link with book icon in footer
- Simplify layout structure
- Remove duplicate code and fix syntax errors
- Add .gitignore for node_modules and other temp files

public/assets/js/app.js

@@ -0,0 +1,7 @@
+// Main application JavaScript
+// This file contains general application functionality
+
+// Initialize application when DOM is ready
+document.addEventListener('DOMContentLoaded', function() {
+    console.log('CodePress CMS initialized');
+});