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/lang/en.php

@@ -35,5 +35,6 @@ return [
     'plugin_development' => 'Plugin Development',
     'template_system' => 'Template System',
     'mqtt_tracking' => 'MQTT Tracking',
-    'real_time_analytics' => 'Real-time Analytics'
+    'real_time_analytics' => 'Real-time Analytics',
+    'go_to' => 'Go to'
 ];

engine/lang/nl.php

@@ -35,5 +35,6 @@ return [
     'plugin_development' => 'Plugin Ontwikkeling',
     'template_system' => 'Template Systeem',
     'mqtt_tracking' => 'MQTT Tracking',
-    'real_time_analytics' => 'Real-time Analytics'
+    'real_time_analytics' => 'Real-time Analytics',
+    'go_to' => 'Ga naar'
 ];