v2.6.0: Content backup/git versioning, plugin type system, docs update

New features:
- ContentBackup class with ZIP backup/restore and git versioning
- Admin backup & restore page (content-backup.twig) with git init/commit/log/restore
- Plugin type system: system (blue) vs content (green) with visual badges
- PluginAPIInterface + AdminPluginAPI for plugin architecture
- Essential plugin flag (cannot edit/deactivate/delete)

Improvements:
- Consolidated enabled_plugins config (removed plugins.enabled)
- Removed Analytics/Logging toggles from admin config page
- Fixed Dashboard plugin Twig comments rendered as text
- Updated 20 guide files (NL+EN): configuratie, plugins, plugin-development,
  core-classes, theme-json, layouts, scss-styling, admin-beheerder, nieuw-thema, architectuur
- Improved accessibility test script (grep -E, min/max checks)

Cleanup:
- Removed unused classes: ARIAComponents, AccessibilityManager, ContentSecurityPolicy, etc.
- Removed vendor packages: mustache/mustache, php-mqtt/client
- Removed old templates: logs.twig, statistics.twig (now plugins)
- Moved language files to language/ directory

Tests:
- Pentest: 30/30 passed, 0 vulnerabilities
- WCAG 2.1 AA: 25/25 passed, 100% compliance

cms/core/index.php

@@ -8,7 +8,7 @@
  * 
  * Architecture:
  * - config.php: Configuration loader that merges default settings with config.json
- * - SimpleTemplate.php: Lightweight template engine for rendering HTML with placeholders
+ * - ThemeManager.php: Twig-based theme rendering + SCSS compilation
  * - CodePressCMS.php: Main CMS class handling content, navigation, search, and rendering
  * 
  * Usage:
@@ -32,14 +32,13 @@ if (file_exists($autoloader)) {
     require_once $autoloader;
 }
 
-// Load template engine - renders HTML with {{variable}} placeholders and conditionals
+// Load core utility classes
 require_once 'class/Cache.php';
 require_once 'class/RateLimiter.php';
 require_once 'class/BotGuard.php';
 require_once 'class/RequestLogger.php';
 require_once 'class/GeoIP.php';
 require_once 'class/Analytics.php';
-require_once 'class/SimpleTemplate.php';
 require_once 'class/ThemeManager.php';
 
 // Load Logger class - structured logging with log levels
@@ -47,7 +46,9 @@ require_once 'class/Logger.php';
 require_once 'class/LogManager.php';
 
 // Load Plugin system
+require_once 'plugin/PluginAPIInterface.php';
 require_once 'plugin/CMSAPI.php';
+require_once 'plugin/AdminPluginAPI.php';
 require_once 'plugin/PluginManager.php';
 
 // Load ContentAPI class - provides CMS data access for PHP content files