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

engine/router.php

@@ -0,0 +1,52 @@
+<?php
+// Router file for PHP development server to handle security and static files
+
+$requestUri = $_SERVER['REQUEST_URI'];
+$parsedUrl = parse_url($requestUri);
+$path = $parsedUrl['path'];
+
+// Block direct access to content directory
+if (strpos($path, '/content/') === 0) {
+    http_response_code(403);
+    echo '<h1>403 - Forbidden</h1><p>Direct access to content files is not allowed.</p>';
+    return true;
+}
+
+// Block access to sensitive files
+$sensitiveFiles = ['.htaccess', 'config.php'];
+foreach ($sensitiveFiles as $file) {
+    if (basename($path) === $file && dirname($path) === '/') {
+        http_response_code(403);
+        echo '<h1>403 - Forbidden</h1><p>Access to this file is not allowed.</p>';
+        return true;
+    }
+}
+
+// Serve static files from engine/assets
+if (strpos($path, '/engine/') === 0) {
+    $filePath = __DIR__ . $path;
+    if (file_exists($filePath)) {
+        // Set appropriate content type
+        $extension = strtolower(pathinfo($filePath, PATHINFO_EXTENSION));
+        $mimeTypes = [
+            'css' => 'text/css',
+            'js' => 'application/javascript',
+            'svg' => 'image/svg+xml',
+            'woff' => 'font/woff',
+            'woff2' => 'font/woff2',
+            'ttf' => 'font/ttf'
+        ];
+        
+        if (isset($mimeTypes[$extension])) {
+            header('Content-Type: ' . $mimeTypes[$extension]);
+        }
+        
+        // Serve the file
+        readfile($filePath);
+        return true;
+    }
+}
+
+// Route all other requests to index.php
+include __DIR__ . '/index.php';
+return true;