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/en/codepress-developer/architectuur.md

@@ -2,10 +2,23 @@
 
 ```
 codepress/
-├── cms/core/           # Core engine
-├── admin/              # Admin console
-├── themes/             # Themes
-├── plugins/            # Plugins
-├── content/            # Content
-└── public/             # Web root
+├── cms/core/                # Core engine
+│   ├── class/               # CodePressCMS, ThemeManager, Logger, Analytics
+│   ├── plugin/              # PluginManager, CMSAPI, AdminPluginAPI
+│   ├── config.php           # Config loader
+│   └── index.php            # Bootstrap
+├── admin/                   # Admin console
+│   ├── config/              # app.php, admin.json
+│   ├── src/                 # AdminAuth
+│   └── theme/default/views/ # Twig templates
+├── themes/                  # Themes (default, demo, ...)
+├── plugins/                 # Plugins (Dashboard, HTMLBlock, Navigation, ...)
+├── content/                 # Content files (.md, .php, .html)
+├── language/                # Language files (nl/, en/, de/)
+├── guide/                   # Guides (nl/, en/)
+├── public/                  # Web root
+│   ├── index.php            # Website entry point
+│   └── admin.php            # Admin entry point + router
+├── config.json              # Site configuration
+└── version.php              # Version info
 ```