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/admin-beheerder/configuratie.md

@@ -3,7 +3,13 @@
 ## Algemene instellingen
 
 - **Site titel** - Naam van de website
-- **Standaard taal** - nl/en/de/fr
-- **Auteur** - Naam en e-mail
-- **Analytics** - Bezoekers tracking aan/uit
-- **Logging** - Logbestanden aan/uit
+- **Admin taal** - Taal van het admin-paneel (nl/en/de)
+- **Content taal** - Standaardtaal van de website-content (nl/en/de/fr)
+
+## Homepage
+
+Kies welke pagina getoond wordt op de homepage:
+
+- **Automatisch** - Eerste beschikbare pagina
+- **Meest recent** - Laatst aangepaste pagina
+- **Specifieke pagina** - Kies een specifieke pagina uit de content-map