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/nl/codepress-developer/core-classes.md

@@ -0,0 +1,32 @@
+# Core Classes
+
+## CodePressCMS.php
+
+Hoofd CMS class in `cms/core/class/CodePressCMS.php`:
+
+```php
+$cms = new CodePressCMS();
+$cms->init();
+$cms->renderPage($pagePath);
+```
+
+## ThemeManager.php
+
+Themabeheer in `cms/core/class/ThemeManager.php`:
+
+```php
+$themeManager = new ThemeManager($config);
+$themeManager->getActiveTheme();
+$themeManager->renderTwig($template, $data);
+$themeManager->compileCss($force);
+```
+
+## PluginManager.php
+
+Plugin systeem in `cms/core/class/PluginManager.php`:
+
+```php
+$pluginManager = new PluginManager();
+$pluginManager->loadPlugins($enabledPlugins);
+$pluginManager->executeHook($name, $params);
+```