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

plugins/Navigation/Navigation.php

@@ -3,17 +3,18 @@
 class Navigation
 {
     private array $config;
-    private ?CMSAPI $api = null;
+    private ?PluginAPIInterface $api = null;
 
     public function __construct()
     {
         $this->config = [
             'title' => 'Navigatie',
             'viewable' => true,
+            'type' => 'content',
         ];
     }
 
-    public function setAPI(CMSAPI $api): void
+    public function setAPI(PluginAPIInterface $api): void
     {
         $this->api = $api;
     }