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/theme.twig

@@ -1,19 +1,19 @@
 {% extends "layouts/admin.twig" %}
 
-{% block title %}Thema's - CodePress Admin{% endblock %}
+{% block title %}{{ ta.themes|default('Thema\'s') }} - {{ ta.admin_title|default('CodePress Admin') }}{% endblock %}
 
 {% block content %}
 <div class="d-flex justify-content-between align-items-center mb-4">
-    <h2><i class="bi bi-palette"></i> Thema's</h2>
+    <h2><i class="bi bi-palette"></i> {{ ta.themes|default('Thema\'s') }}</h2>
     <div>
         <form method="POST" action="/admin/theme" class="d-inline">
             <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
             <button type="submit" name="activate_default" class="btn btn-primary btn-sm">
-                <i class="bi bi-check-lg"></i> Activeer Default
+                <i class="bi bi-check-lg"></i> {{ ta.activate_default|default('Activeer Default') }}
             </button>
         </form>
         <a href="/admin/theme-new" class="btn btn-outline-primary btn-sm ms-2">
-            <i class="bi bi-plus-lg"></i> Nieuw thema
+            <i class="bi bi-plus-lg"></i> {{ ta.new_theme|default('Nieuw thema') }}
         </a>
     </div>
 </div>
@@ -25,13 +25,13 @@
             <div class="card-header d-flex justify-content-between align-items-center">
                 <span class="fw-bold">{{ theme.title|default(theme.name) }}</span>
                 {% if theme.active %}
-                <span class="badge bg-success">Actief</span>
+                <span class="badge bg-success">{{ ta.active|default('Actief') }}</span>
                 {% endif %}
             </div>
             <div class="card-body">
-                <p class="text-muted small">Naam: {{ theme.name }}</p>
+                <p class="text-muted small">{{ ta.name_label|default('Naam: ') }}{{ theme.name }}</p>
                 {% if theme.default_layout %}
-                <p class="text-muted small">Default layout: {{ theme.default_layout }}</p>
+                <p class="text-muted small">{{ ta.default_layout_label|default('Default layout: ') }}{{ theme.default_layout }}</p>
                 {% endif %}
             </div>
             <div class="card-footer bg-white">
@@ -40,7 +40,7 @@
                     <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
                     <input type="hidden" name="activate" value="{{ theme.name }}">
                     <button type="submit" class="btn btn-sm btn-outline-primary">
-                        <i class="bi bi-check-lg"></i> Activeren
+                        <i class="bi bi-check-lg"></i> {{ ta.activate|default('Activeren') }}
                     </button>
                 </form>
                 {% endif %}
@@ -49,7 +49,7 @@
                     <input type="hidden" name="compile_scss" value="1">
                     <input type="hidden" name="theme" value="{{ theme.name }}">
                     <button type="submit" class="btn btn-sm btn-outline-success">
-                        <i class="bi bi-palette"></i> SCSS compileren
+                        <i class="bi bi-palette"></i> {{ ta.compile_scss|default('SCSS compileren') }}
                     </button>
                 </form>
             </div>
@@ -58,9 +58,9 @@
     {% else %}
     <div class="col-12">
         <div class="alert alert-info">
-            <i class="bi bi-info-circle"></i> Geen thema's gevonden.
+            <i class="bi bi-info-circle"></i> {{ ta.no_themes|default('Geen thema\'s gevonden.') }}
         </div>
     </div>
     {% endfor %}
 </div>
-{% endblock %}
+{% endblock %}