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/config/admin.json.example

@@ -0,0 +1,21 @@
+{
+    "users": [
+        {
+            "username": "admin",
+            "password_hash": "$2y$12$CV2uORTIiie6s3jf9ANaneL7VLNqA5smXyrIzkcJFjlS2Vxfw1/HK",
+            "role": "admin",
+            "email": "",
+            "author_name": "",
+            "author_email": "",
+            "created": "2025-01-01"
+        }
+    ],
+    "security": {
+        "session_timeout": 1800,
+        "max_login_attempts": 5,
+        "lockout_duration": 900
+    },
+    "config": {
+        "default_page": "auto"
+    }
+}