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

vendor/composer/autoload_psr4.php

@@ -16,10 +16,8 @@ return array(
     'Psr\\Log\\' => array($vendorDir . '/psr/log/src'),
     'Psr\\Http\\Message\\' => array($vendorDir . '/psr/http-factory/src', $vendorDir . '/psr/http-message/src'),
     'Psr\\EventDispatcher\\' => array($vendorDir . '/psr/event-dispatcher/src'),
-    'PhpMqtt\\Client\\' => array($vendorDir . '/php-mqtt/client/src'),
     'Nette\\' => array($vendorDir . '/nette/schema/src', $vendorDir . '/nette/utils/src'),
     'MyCLabs\\Enum\\' => array($vendorDir . '/myclabs/php-enum/src'),
-    'Mustache\\' => array($vendorDir . '/mustache/mustache/src'),
     'MaxMind\\WebService\\' => array($vendorDir . '/maxmind/web-service-common/src/WebService'),
     'MaxMind\\Exception\\' => array($vendorDir . '/maxmind/web-service-common/src/Exception'),
     'MaxMind\\Db\\' => array($vendorDir . '/maxmind-db/reader/src/MaxMind/Db'),