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/admin-beheerder/thema-beheer.md

@@ -0,0 +1,21 @@
+# Theme management
+
+## Managing themes
+
+1. Go to **Theme** in admin menu
+2. **Activate** - Choose active theme
+3. **Compile SCSS** - Process SCSS to CSS
+4. **New theme** - Create custom theme
+
+## Theme structure
+
+```
+themes/default/
+├── theme.json          # Theme configuration
+├── base.twig           # Main layout
+├── full_content.twig   # Layouts
+├── left_sidebar.twig
+├── right_sidebar.twig
+├── partials/           # Header, nav, footer
+└── assets/             # CSS, JS, images
+```