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

guide/en/content-beheerder/content-api.md

@@ -0,0 +1,119 @@
+# Content API
+
+The Content API is available in PHP content files (`.php`) and provides a safe, read-only interface to CMS data.
+
+## Usage in PHP content files
+
+```php
+<?php
+/** @var ContentAPI $api */
+
+// Get all pages
+$allPages = $api->getAllPages();
+// Result: ['index' => 'Home', 'about-us' => 'About Us', ...]
+
+// Get a specific page
+$page = $api->getPage('about-us');
+// Result: ['title' => 'About Us', 'content' => '...', 'path' => 'about-us', 'layout' => 'full_content', 'metadata' => [...]]
+
+// Get menu structure
+$menu = $api->getMenu();
+// Result: [['title' => 'Home', 'path' => 'index', 'type' => 'file', 'active' => true], ...]
+
+// Get config value (dot notation)
+$siteTitle = $api->getConfig('site_title');
+$searchEnabled = $api->getConfig('features.search_enabled', false);
+```
+
+## Available methods
+
+### Pages
+
+- `getAllPages(): array` - All pages as `['path' => 'title']` pairs
+- `getPage(string $path): ?array` - Specific page with `title`, `content`, `path`, `layout`, `metadata`
+- `pageExists(string $path): bool` - Check if a page exists
+- `getCurrentPageTitle(): string` - Title of current page
+- `getCurrentPagePath(): string` - Path of current page
+- `isHomepage(): bool` - Whether current page is the homepage
+
+### Menu & Navigation
+
+- `getMenu(): array` - Hierarchical menu structure with `title`, `path`, `children`, `active`
+- `buildUrl(string $page = 'index', ?string $lang = null, array $params = []): string` - Build a URL for a page
+
+### Configuration
+
+- `getConfig(string $key, mixed $default = null): mixed` - Config value via dot notation (e.g. `'features.search_enabled'`)
+- `getSiteTitle(): string` - Site title from config
+
+### Language
+
+- `getCurrentLanguage(): string` - Current language code (e.g. `'nl'`)
+- `getAvailableLanguages(): array` - Available languages (e.g. `['nl', 'en']`)
+- `t(string $key): string` - Translate a language key
+
+### Search
+
+- `getSearchResults(): array` - Search results (empty if not searching)
+- `isSearching(): bool` - Whether a search is currently active
+
+## Example: Show recent pages
+
+```php
+<?php
+/** @var ContentAPI $api */
+$pages = $api->getAllPages();
+$currentLang = $api->getCurrentLanguage();
+?>
+<ul>
+<?php foreach (array_slice($pages, 0, 5, true) as $path => $title): ?>
+    <li>
+        <a href="/<?= $currentLang ?>/<?= htmlspecialchars($path) ?>">
+            <?= htmlspecialchars($title) ?>
+        </a>
+    </li>
+<?php endforeach; ?>
+</ul>
+```
+
+## Example: Using config values
+
+```php
+<?php
+/** @var ContentAPI $api */
+if ($api->getConfig('features.search_enabled', false)): ?>
+    <form method="GET" action="">
+        <input type="search" name="search" placeholder="<?= $api->t('search_placeholder') ?>">
+        <button type="submit"><?= $api->t('search_button') ?></button>
+    </form>
+<?php endif; ?>
+```
+
+## CMSAPI (for plugins)
+
+Plugins use the `CMSAPI` class via `$this->api`. It offers similar methods:
+
+```php
+$this->api->getCurrentPageTitle();
+$this->api->getCurrentPageContent();
+$this->api->getMenu();
+$this->api->getConfig('site_title');
+$this->api->getCurrentLanguage();
+$this->api->isHomepage();
+$this->api->hasContent();
+$this->api->getSearchResults();
+$this->api->isSearching();
+$this->api->getAvailableLanguages();
+$this->api->createUrl('about-us');
+$this->api->translate('home');
+```
+
+Additionally, CMSAPI provides:
+
+- `getCurrentPage(): array` - Full page data
+- `getCurrentPageUrl(): string` - URL of current page
+- `getCurrentPageFileInfo(): ?array` - File info (created, modified)
+- `getBreadcrumb(): string` - Breadcrumb HTML
+- `executePhpFile(string $filePath): string` - Execute PHP file and capture output
+- `getFileContent(string $filePath): string` - Get content from PHP/HTML/Markdown file
+- `contentFileExists(string $filename): bool` - Check if file exists in content directory