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

language/nl/site.php

@@ -0,0 +1,38 @@
+<?php
+return [
+    'site_title' => 'CodePress',
+    'home' => 'Home',
+    'search' => 'Zoeken',
+    'search_placeholder' => 'Zoeken...',
+    'search_button' => 'Zoeken',
+    'welcome' => 'Welkom',
+    'created' => 'Aangemaakt',
+    'modified' => 'Aangepast',
+    'author' => 'Auteur',
+    'manual' => 'Handleiding',
+    'no_content' => 'Geen inhoud gevonden',
+    'no_results' => 'Geen resultaten gevonden',
+    'results_found' => 'resultaten gevonden',
+    'breadcrumb_home' => 'Home',
+    'file_details' => 'Bestandsdetails',
+    'guide' => 'Handleiding',
+    'powered_by' => 'Mogelijk gemaakt door',
+    't_powered_by' => 'Mogelijk gemaakt door',
+    'directory_empty' => 'Deze map is leeg',
+    'page_not_found' => 'Pagina niet gevonden',
+    'page_not_found_text' => 'De pagina die u zoekt bestaat niet.',
+    'mappen' => 'Mappen',
+    'paginas' => 'Pagina\'s',
+    'author_website' => 'Auteur website',
+    'author_git' => 'Auteur Git',
+    'plugins' => 'Plugins',
+    'templates' => 'Templates',
+    'layouts' => 'Layouts',
+    'sidebar_content' => 'Sidebar + Content',
+    'content_only' => 'Alleen Content',
+    'sidebar_only' => 'Alleen Sidebar',
+    'content_sidebar' => 'Content + Sidebar',
+    'plugin_development' => 'Plugin Ontwikkeling',
+    'template_system' => 'Template Systeem',
+    'go_to' => 'Ga naar',
+];