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/media.twig

@@ -1,12 +1,12 @@
 {% extends "layouts/admin.twig" %}
 
-{% block title %}Media - CodePress Admin{% endblock %}
+{% block title %}{{ ta.media|default('Media') }} - {{ 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-images"></i> Media</h2>
+    <h2><i class="bi bi-images"></i> {{ ta.media|default('Media') }}</h2>
     <button type="button" class="btn btn-primary btn-sm" data-bs-toggle="collapse" data-bs-target="#uploadForm">
-        <i class="bi bi-cloud-upload"></i> Uploaden
+        <i class="bi bi-cloud-upload"></i> {{ ta.upload|default('Upload') }}
     </button>
 </div>
 
@@ -16,11 +16,11 @@
             <form method="POST" action="/admin/media?dir={{ subdir|url_encode }}" enctype="multipart/form-data">
                 <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
                 <div class="mb-3">
-                    <label for="file" class="form-label">Bestanden selecteren</label>
+                    <label for="file" class="form-label">{{ ta.select_files|default('Bestanden selecteren') }}</label>
                     <input type="file" class="form-control" id="file" name="file[]" multiple accept="image/*,video/*,audio/*">
                 </div>
                 <button type="submit" class="btn btn-success">
-                    <i class="bi bi-cloud-upload"></i> Uploaden
+                    <i class="bi bi-cloud-upload"></i> {{ ta.upload|default('Uploaden') }}
                 </button>
             </form>
         </div>
@@ -43,10 +43,10 @@
                 {% endif %}
             {% else %}
             <div class="col-12">
-                <p class="text-muted text-center">Geen media bestanden gevonden.</p>
+                <p class="text-muted text-center">{{ ta.no_media|default('Geen media bestanden gevonden.') }}</p>
             </div>
             {% endfor %}
         </div>
     </div>
 </div>
-{% endblock %}
+{% endblock %}