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/admin-beheerder/plugins.md

@@ -0,0 +1,15 @@
+# Plugins
+
+## Plugins beheren
+
+- **Activeren/Deactiveren** - Plugins aan/uit
+- **Bewerken** - Plugin code aanpassen
+- **Configuratie** - Plugin instellingen
+- **Verwijderen** - Plugin verwijderen
+
+## Nieuwe plugin
+
+1. Ga naar **Plugins** → **Nieuwe plugin**
+2. Geef naam op (bijv. `MijnPlugin`)
+3. Bewerk `plugin.php`
+4. Activeer de plugin