Fix security vulnerabilities, remove dead code, and improve code quality

- Fix path traversal with realpath() validation in getPage() and executePhpFile()
- Remove insecure JWT secret fallback, require JWT_SECRET env var
- Fix IP spoofing by only trusting proxy headers from configured proxies
- Add Secure/HttpOnly/SameSite flags to all cookies
- Use env var for debug mode instead of hardcoded true
- Fix operator precedence bug in MQTTTracker track_user_flows check
- Remove dead code: duplicate is_dir() block, unused scanForPageNames()
- Remove htmlspecialchars() from filesystem path operations
- Remove duplicate require_once calls and redundant autoloader includes
- Fix unclosed </div> in getDirectoryListing()
- Escape breadcrumb titles and add lang param to search result URLs
- Make language prefixes dynamic from config instead of hardcoded nl|en
- Make HTML lang attribute dynamic, add go_to translation key
- Add aria-label/aria-expanded to sidebar toggle for accessibility
- Fix event listener leak in app.js using event delegation
- Remove console.log from production code
- Update guides (NL/EN) with sidebar toggle documentation
- Add TODO.md documenting all identified improvements

engine/core/plugin/CMSAPI.php

@@ -167,6 +167,13 @@ class CMSAPI
             return '';
         }
         
+        // Validate file is within the CMS directory to prevent arbitrary file inclusion
+        $realPath = realpath($filePath);
+        $cmsRoot = realpath(__DIR__ . '/../../../');
+        if (!$realPath || !$cmsRoot || strpos($realPath, $cmsRoot) !== 0) {
+            return '';
+        }
+        
         ob_start();
         include $filePath;
         return ob_get_clean();