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-new.twig

@@ -1,24 +1,24 @@
 {% extends "layouts/admin.twig" %}
 
-{% block title %}Nieuw thema - CodePress Admin{% endblock %}
+{% block title %}{{ ta.new_theme|default('Nieuw thema') }} - {{ ta.admin_title|default('CodePress Admin') }}{% endblock %}
 
 {% block content %}
-<h2 class="mb-4"><i class="bi bi-palette"></i> Nieuw thema aanmaken</h2>
+<h2 class="mb-4"><i class="bi bi-palette"></i> {{ ta.new_theme_title|default('Nieuw thema aanmaken') }}</h2>
 
 <form method="post" class="card shadow-sm">
     <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
     <div class="card-body">
         <div class="mb-3">
-            <label for="name" class="form-label">Thema naam</label>
+            <label for="name" class="form-label">{{ ta.theme_name|default('Thema naam') }}</label>
             <input type="text" class="form-control" id="name" name="name" required autofocus>
-            <small class="form-text text-muted">Alleen letters, cijfers, underscores en streepjes.</small>
+            <small class="form-text text-muted">{{ ta.theme_name_help|default('Alleen letters, cijfers, underscores en streepjes.') }}</small>
         </div>
     </div>
     <div class="card-footer bg-white">
         <button type="submit" class="btn btn-primary">
-            <i class="bi bi-check-lg"></i> Aanmaken
+            <i class="bi bi-check-lg"></i> {{ ta.create|default('Aanmaken') }}
         </button>
-        <a href="/admin/theme" class="btn btn-outline-secondary">Annuleren</a>
+        <a href="/admin/theme" class="btn btn-outline-secondary">{{ ta.cancel|default('Annuleren') }}</a>
     </div>
 </form>
-{% endblock %}
+{% endblock %}