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/theme-developer/thema-structuur.md

@@ -0,0 +1,21 @@
+# Theme structure
+
+```
+themes/my-theme/
+├── theme.json              # Theme configuration
+├── base.twig               # Main layout
+├── full_content.twig       # Layout: full-width
+├── left_sidebar.twig       # Layout: left sidebar
+├── right_sidebar.twig      # Layout: right sidebar
+├── custom.twig             # Layout: custom
+├── partials/
+│   ├── header.twig
+│   ├── navigation.twig
+│   └── footer.twig
+└── assets/
+    ├── scss/theme.scss     # SCSS source
+    ├── css/                # CSS files
+    ├── js/theme.js         # JavaScript
+    ├── fonts/              # Fonts
+    └── img/                # Images
+```