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/thema-beheer.md

@@ -0,0 +1,19 @@
+# Thema beheer
+
+## Thema's beheren
+
+1. Ga naar **Thema** in admin menu
+2. **Activeren** - Kies actief thema
+3. **SCSS compileren** - Verwerk SCSS naar CSS
+4. **Nieuw thema** - Eigen thema aanmaken
+
+## Thema structuur
+
+```
+themes/default/
+├── theme.json          # Thema configuratie
+├── base.twig           # Hoofd layout
+├── full_content.twig   # Layouts
+├── partials/           # Header, nav, footer
+└── assets/             # CSS, JS, afbeeldingen
+```