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/content-beheerder/content-structuur.md

@@ -0,0 +1,31 @@
+# Content structure
+
+Content is stored in the `content/` folder without a database.
+
+## Supported file formats
+
+- `.md` - Markdown (recommended)
+- `.php` - Dynamic PHP pages
+- `.html` - Static HTML pages
+
+## File name conventions
+
+```
+en.page-name.md     # English page
+nl.pagina-naam.md   # Dutch page
+```
+
+## Frontmatter
+
+Markdown files can contain frontmatter metadata:
+
+```markdown
+---
+layout: full_content
+plugins: HTMLBlock
+---
+
+# Page title
+
+Content...
+```