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

cms/core/plugin/AdminPluginAPI.php

@@ -0,0 +1,82 @@
+<?php
+
+/**
+ * Lightweight API wrapper for plugins in the admin context.
+ * Provides read-only access to the site config (no CMS instance needed).
+ */
+class AdminPluginAPI implements PluginAPIInterface
+{
+    private array $config;
+    private string $projectRoot;
+
+    public function __construct(array $siteConfig, string $projectRoot = '')
+    {
+        $this->config = $siteConfig;
+        $this->projectRoot = $projectRoot !== '' ? $projectRoot : dirname(__DIR__, 3);
+    }
+
+    /**
+     * Get configuration value using dot notation.
+     */
+    public function getConfig(string $key, $default = null)
+    {
+        $keys = explode('.', $key);
+        $value = $this->config;
+
+        foreach ($keys as $k) {
+            if (!is_array($value) || !isset($value[$k])) {
+                return $default;
+            }
+            $value = $value[$k];
+        }
+
+        return $value;
+    }
+
+    /**
+     * Get the project root directory.
+     */
+    public function getProjectRoot(): string
+    {
+        return $this->projectRoot;
+    }
+
+    /**
+     * Get the content directory path.
+     */
+    public function getContentDir(): string
+    {
+        return $this->config['content_dir'] ?? ($this->projectRoot . '/content');
+    }
+
+    /**
+     * Get the plugins directory path.
+     */
+    public function getPluginsDir(): string
+    {
+        return $this->projectRoot . '/plugins';
+    }
+
+    /**
+     * Get the list of enabled plugins.
+     */
+    public function getEnabledPlugins(): array
+    {
+        return $this->config['enabled_plugins'] ?? [];
+    }
+
+    /**
+     * Get the version info from version.php.
+     */
+    public function getVersionInfo(): array
+    {
+        $versionFile = $this->projectRoot . '/version.php';
+        if (file_exists($versionFile)) {
+            $data = include $versionFile;
+            if (is_array($data)) {
+                return $data;
+            }
+        }
+        return ['version' => '0.0.0'];
+    }
+}