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/content-beheerder.md

@@ -0,0 +1,10 @@
+# Content Beheerder
+
+De content beheerder handleiding bevat de volgende onderwerpen:
+
+- **Content structuur** - Hoe content wordt opgeslagen
+- **Pagina's beheren** - Pagina's aanmaken en bewerken
+- **Media beheren** - Media uploaden en gebruiken
+- **Content API** - De Content API gebruiken in PHP pagina's
+
+Selecteer een onderwerp uit de navigatie aan de linkerkant.