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/guide.twig

@@ -1,6 +1,6 @@
 {% extends "layouts/admin.twig" %}
 
-{% block title %}{{ guide_page ? 'Handleiding - ' : '' }}Handleiding{% endblock %}
+{% block title %}{{ guide_page ? (ta.guide|default('Handleiding')) ~ ' - ' : '' }}{{ ta.guide|default('Handleiding') }}{% endblock %}
 
 {% block content %}
 <div class="row">
@@ -8,7 +8,7 @@
     <aside class="col-md-3 mb-3">
         <div class="card shadow-sm">
             <div class="card-header">
-                <h5 class="mb-0"><i class="bi bi-list-ul"></i> Navigatie</h5>
+                <h5 class="mb-0"><i class="bi bi-list-ul"></i> {{ ta.navigation|default('Navigatie') }}</h5>
             </div>
             <div class="card-body">
                 {{ guide_nav|raw }}
@@ -23,9 +23,9 @@
             <ol class="breadcrumb">
                 <li class="breadcrumb-item {{ not guide_page ? 'active' : '' }}">
                     {% if guide_page %}
-                        <a href="/admin/guide{% if guide_lang %}?lang={{ guide_lang }}{% endif %}">Handleiding</a>
+                        <a href="/admin/guide{% if guide_lang %}?lang={{ guide_lang }}{% endif %}">{{ ta.guide|default('Handleiding') }}</a>
                     {% else %}
-                        Handleidingen
+                        {{ ta.manuals|default('Handleidingen') }}
                     {% endif %}
                 </li>
                 {% if guide_breadcrumbs %}