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

README.md

@@ -63,8 +63,8 @@ codepress/
 ├── cms/                        # Core CMS engine
 │   ├── core/class/             # CMS classes (CodePressCMS, ThemeManager, etc.)
 │   ├── core/plugin/            # Plugin systeem (PluginManager, CMSAPI)
-│   ├── lang/                   # Taalbestanden (nl.php, en.php)
 │   └── router.php              # PHP dev server router (schone URLs)
+├── language/                   # Taalbestanden (nl/, en/, de/ — elk met site.php + admin.php)
 ├── admin/                      # Admin console
 │   ├── config/                 # Admin configuratie (admin.json)
 │   ├── src/AdminAuth.php       # Authenticatie, rollen, permissies