v2.5.1: Admin theme refactor, Navigation plugin, user roles, guide restructure

- Reorganize admin into admin/theme/default/ (views + assets)
- Rename GuideNav to Navigation plugin (essential, protected)
- Plugin assets support (SCSS/CSS) loaded after theme CSS
- User roles: Admin, Content Manager, BI Manager, Site Admin
- Role-based access control (RBAC) for admin routes and sidebar
- Guide restructure: sub-topics in separate folders with sidebar nav
- Dynamic breadcrumb for homepage and subdirectories
- Fix theme path traversal (../../ -> ../) in admin.php
- Fix CodeMirror mode load order (xml -> css -> js -> htmlmixed -> php)
- Fix editor-toolbar.js null checks for plugin edit pages
- Layout select from theme.json with live frontmatter update
- Footer sticky at bottom of viewport (min-height: 100vh)
- Breadcrumb color fix (var(--nav-font) -> var(--header-bg))
- Remove language switcher from guide pages
- Update README.md and README.en.md
- Bump version to 2.5.1

public/test-guide.php

@@ -0,0 +1,20 @@
+<?php
+require_once __DIR__ . '/../vendor/autoload.php';
+
+$rootDir = dirname(__DIR__);
+$lang = 'nl';
+$pagePath = '';
+
+$guideFile = $rootDir . '/guide/' . $lang . '/index.md';
+
+echo "File: $guideFile\n";
+echo "Exists: " . (file_exists($guideFile) ? 'YES' : 'NO') . "\n\n";
+
+if (file_exists($guideFile)) {
+    $content = file_get_contents($guideFile);
+    $environment = new \League\CommonMark\Environment\Environment(['html_input' => 'strip']);
+    $environment->addExtension(new \League\CommonMark\Extension\CommonMark\CommonMarkCoreExtension());
+    $environment->addExtension(new \League\CommonMark\Extension\Table\TableExtension());
+    $converter = new \League\CommonMark\MarkdownConverter($environment);
+    echo $converter->convert($content)->getContent();
+}