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/plugins-edit.twig

@@ -1,6 +1,6 @@
 {% extends "layouts/admin.twig" %}
 
-{% block title %}Plugin bewerken: {{ pluginName }} - CodePress Admin{% endblock %}
+{% block title %}{{ ta.plugin_edit|default('Plugin bewerken: ') }}{{ pluginName }} - {{ ta.admin_title|default('CodePress Admin') }}{% endblock %}
 
 {% block extra_css %}
 <link rel="stylesheet" href="/admin/assets/codemirror/codemirror.min.css">
@@ -9,9 +9,9 @@
 
 {% block content %}
 <div class="d-flex justify-content-between align-items-center mb-4">
-    <h2><i class="bi bi-pencil"></i> Plugin bewerken: {{ pluginName }}</h2>
+    <h2><i class="bi bi-pencil"></i> {{ ta.plugin_edit|default('Plugin bewerken: ') }}{{ pluginName }}</h2>
     <a href="/admin/plugins" class="btn btn-outline-secondary btn-sm">
-        <i class="bi bi-arrow-left"></i> Terug
+        <i class="bi bi-arrow-left"></i> {{ ta.back|default('Terug') }}
     </a>
 </div>
 
@@ -27,12 +27,12 @@
     </div>
     <div class="d-flex gap-2 mt-3">
         <button type="submit" class="btn btn-primary">
-            <i class="bi bi-check-lg"></i> Opslaan
+            <i class="bi bi-check-lg"></i> {{ ta.save|default('Opslaan') }}
         </button>
-        <a href="/admin/plugins" class="btn btn-outline-secondary">Annuleren</a>
+        <a href="/admin/plugins" class="btn btn-outline-secondary">{{ ta.cancel|default('Annuleren') }}</a>
     </div>
 </form>
 {% endblock %}
 
 {% block extra_js %}
-{% endblock %}
+{% endblock %}