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

guide/nl/theme-developer/layouts.md

@@ -16,12 +16,17 @@ Content...
 
 ```json
 {
-  "default_layout": "full_content",
-  "layouts": {
+  "config": {
+    "default_template": "full_content"
+  },
+  "template": {
     "full_content": "full_content.twig",
     "left_sidebar": "left_sidebar.twig",
     "right_sidebar": "right_sidebar.twig",
-    "custom1": "custom1.twig"
+    "custom1": "custom1.twig",
+    "guide": "guide.twig"
   }
 }
-```
+```
+
+Onbekende layouts in frontmatter vallen terug op `config.default_template`.