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/update.twig

@@ -1,35 +1,35 @@
 {% extends "layouts/admin.twig" %}
 
-{% block title %}Update - CodePress Admin{% endblock %}
+{% block title %}{{ ta.update|default('Update') }} - {{ ta.admin_title|default('CodePress Admin') }}{% endblock %}
 
 {% block content %}
-<h2 class="mb-4"><i class="bi bi-cloud-arrow-down"></i> Systeem Update</h2>
+<h2 class="mb-4"><i class="bi bi-cloud-arrow-down"></i> {{ ta.system_update|default('Systeem Update') }}</h2>
 
 {% if isGitWritable == false %}
     <div class="alert alert-warning mb-4">
         <i class="bi bi-exclamation-triangle-fill me-1"></i>
-        De .git map is niet beschrijfbaar. Automatische updates zijn niet mogelijk.
+        {{ ta.git_not_writable|default('De .git map is niet beschrijfbaar. Automatische updates zijn niet mogelijk.') }}
     </div>
 {% endif %}
 
 <div class="card shadow-sm mb-4">
-    <div class="card-header">Huidige versie</div>
+    <div class="card-header">{{ ta.current_version|default('Huidige versie') }}</div>
     <div class="card-body">
-        <p class="mb-0">CodePress versie: <strong>{{ version }}</strong></p>
+        <p class="mb-0">{{ ta.cms_version_label|default('CodePress versie: ') }}<strong>{{ version }}</strong></p>
     </div>
 </div>
 
 <div class="card shadow-sm">
-    <div class="card-header">Update opties</div>
+    <div class="card-header">{{ ta.update_options|default('Update opties') }}</div>
     <div class="card-body">
         <div class="d-grid gap-2">
             <button class="btn btn-primary" {{ isGitWritable ? '' : 'disabled' }}>
-                <i class="bi bi-cloud-arrow-down"></i> Controleer op updates
+                <i class="bi bi-cloud-arrow-down"></i> {{ ta.check_updates|default('Controleer op updates') }}
             </button>
             <button class="btn btn-outline-secondary" {{ isGitWritable ? '' : 'disabled' }}>
-                <i class="bi bi-arrow-repeat"></i> Update uitvoeren
+                <i class="bi bi-arrow-repeat"></i> {{ ta.run_update|default('Update uitvoeren') }}
             </button>
         </div>
     </div>
 </div>
-{% endblock %}
+{% endblock %}