Implement modern sidebar navigation with hamburger menu

- Add responsive sidebar with hamburger toggle functionality
- Implement dual toggle buttons (inner/outer) for better UX
- Fix sidebar positioning to not overlap header and footer
- Add sticky footer with proper z-index layering
- Download and integrate Bootstrap source maps locally
- Optimize toggle icons: smaller, cleaner, no button styling
- Ensure sidebar respects footer boundaries
- Add smooth transitions and hover effects
- Fix active page highlighting and folder auto-expansion
- Create professional W3Schools-style navigation
- Maintain full offline capability with local assets

public/index.php

@@ -438,7 +438,7 @@ class CodePressCMS {
 }
 
 // Block direct access to content files
-$requestUri = $_SERVER['REQUEST_URI'];
+$requestUri = $_SERVER['REQUEST_URI'] ?? '';
 if (strpos($requestUri, '/content/') !== false) {
     http_response_code(403);
     echo '<h1>403 - Forbidden</h1><p>Direct access to content files is not allowed.</p>';