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

admin/theme/default/views/pages/error.twig

@@ -1,14 +1,14 @@
 {% extends "layouts/admin.twig" %}
 
-{% block title %}{{ error_title|default('Fout') }} - CodePress Admin{% endblock %}
+{% block title %}{{ error_title|default(ta.error|default('Fout')) }} - {{ ta.admin_title|default('CodePress Admin') }}{% endblock %}
 
 {% block content %}
 <div class="text-center py-5">
     <h1 class="display-1 text-muted">{{ error_code|default('404') }}</h1>
-    <h2 class="mb-3">{{ error_title|default('Pagina niet gevonden') }}</h2>
-    <p class="text-muted mb-4">{{ error_message|default('De gevraagde pagina kon niet worden gevonden.') }}</p>
+    <h2 class="mb-3">{{ error_title|default(ta.page_not_found|default('Pagina niet gevonden')) }}</h2>
+    <p class="text-muted mb-4">{{ error_message|default(ta.page_not_found_msg|default('De gevraagde pagina kon niet worden gevonden.')) }}</p>
     <a href="/admin/dashboard" class="btn btn-primary">
-        <i class="bi bi-house"></i> Naar dashboard
+        <i class="bi bi-house"></i> {{ ta.to_dashboard|default('Naar dashboard') }}
     </a>
 </div>
 {% endblock %}